On Mon, Mar 16, 2026 at 6:39 AM Lorenzo Stoakes (Oracle) <[email protected]> wrote: > > On Sun, Mar 15, 2026 at 07:18:38PM -0700, Suren Baghdasaryan wrote: > > On Fri, Mar 13, 2026 at 4:58 AM Lorenzo Stoakes (Oracle) <[email protected]> > > wrote: > > > > > > On Fri, Mar 13, 2026 at 04:02:36AM -0700, Usama Arif wrote: > > > > On Thu, 12 Mar 2026 20:27:19 +0000 "Lorenzo Stoakes (Oracle)" > > > > <[email protected]> wrote: > > > > > > > > > Previously, when a driver needed to do something like establish a > > > > > reference > > > > > count, it could do so in the mmap hook in the knowledge that the > > > > > mapping > > > > > would succeed. > > > > > > > > > > With the introduction of f_op->mmap_prepare this is no longer the > > > > > case, as > > > > > it is invoked prior to actually establishing the mapping. > > > > > > > > > > To take this into account, introduce a new vm_ops->mapped callback > > > > > which is > > > > > invoked when the VMA is first mapped (though notably - not when it is > > > > > merged - which is correct and mirrors existing mmap/open/close > > > > > behaviour). > > > > > > > > > > We do better that vm_ops->open() here, as this callback can return an > > > > > error, at which point the VMA will be unmapped. > > > > > > > > > > Note that vm_ops->mapped() is invoked after any mmap action is > > > > > complete (such as I/O remapping). > > > > > > > > > > We intentionally do not expose the VMA at this point, exposing only > > > > > the > > > > > fields that could be used, and an output parameter in case the > > > > > operation > > > > > needs to update the vma->vm_private_data field. > > > > > > > > > > In order to deal with stacked filesystems which invoke inner > > > > > filesystem's > > > > > mmap() invocations, add __compat_vma_mapped() and invoke it on > > > > > vfs_mmap() (via compat_vma_mmap()) to ensure that the mapped callback > > > > > is > > > > > handled when an mmap() caller invokes a nested filesystem's > > > > > mmap_prepare() > > > > > callback. > > > > > > > > > > We can now also remove call_action_complete() and invoke > > > > > mmap_action_complete() directly, as we separate out the rmap lock > > > > > logic to > > > > > be called in __mmap_region() instead via maybe_drop_file_rmap_lock(). > > > > > > > > > > We also abstract unmapping of a VMA on mmap action completion into > > > > > its own > > > > > helper function, unmap_vma_locked(). > > > > > > > > > > Additionally, update VMA userland test headers to reflect the change. > > > > > > > > > > Signed-off-by: Lorenzo Stoakes (Oracle) <[email protected]> > > > > > --- > > > > > include/linux/fs.h | 9 +++- > > > > > include/linux/mm.h | 17 +++++++ > > > > > mm/internal.h | 10 ++++ > > > > > mm/util.c | 86 > > > > > ++++++++++++++++++++++++--------- > > > > > mm/vma.c | 41 +++++++++++----- > > > > > tools/testing/vma/include/dup.h | 34 ++++++++++++- > > > > > 6 files changed, 158 insertions(+), 39 deletions(-) > > > > > > > > > > diff --git a/include/linux/fs.h b/include/linux/fs.h > > > > > index a2628a12bd2b..c390f5c667e3 100644 > > > > > --- a/include/linux/fs.h > > > > > +++ b/include/linux/fs.h > > > > > @@ -2059,13 +2059,20 @@ static inline bool can_mmap_file(struct file > > > > > *file) > > > > > } > > > > > > > > > > int compat_vma_mmap(struct file *file, struct vm_area_struct *vma); > > > > > +int __vma_check_mmap_hook(struct vm_area_struct *vma); > > > > > > > > > > static inline int vfs_mmap(struct file *file, struct vm_area_struct > > > > > *vma) > > > > > { > > > > > + int err; > > > > > + > > > > > if (file->f_op->mmap_prepare) > > > > > return compat_vma_mmap(file, vma); > > > > > > > > > > - return file->f_op->mmap(file, vma); > > > > > + err = file->f_op->mmap(file, vma); > > > > > + if (err) > > > > > + return err; > > > > > + > > > > > + return __vma_check_mmap_hook(vma); > > > > > } > > > > > > > > > > static inline int vfs_mmap_prepare(struct file *file, struct > > > > > vm_area_desc *desc) > > > > > diff --git a/include/linux/mm.h b/include/linux/mm.h > > > > > index 12a0b4c63736..7333d5db1221 100644 > > > > > --- a/include/linux/mm.h > > > > > +++ b/include/linux/mm.h > > > > > @@ -759,6 +759,23 @@ struct vm_operations_struct { > > > > > * Context: User context. May sleep. Caller holds mmap_lock. > > > > > */ > > > > > void (*close)(struct vm_area_struct *vma); > > > > > + /** > > > > > + * @mapped: Called when the VMA is first mapped in the MM. Not > > > > > called if > > > > > + * the new VMA is merged with an adjacent VMA. > > > > > + * > > > > > + * The @vm_private_data field is an output field allowing the > > > > > user to > > > > > + * modify vma->vm_private_data as necessary. > > > > > + * > > > > > + * ONLY valid if set from f_op->mmap_prepare. Will result in an > > > > > error if > > > > > + * set from f_op->mmap. > > > > > + * > > > > > + * Returns %0 on success, or an error otherwise. On error, the > > > > > VMA will > > > > > + * be unmapped. > > > > > + * > > > > > + * Context: User context. May sleep. Caller holds mmap_lock. > > > > > + */ > > > > > + int (*mapped)(unsigned long start, unsigned long end, pgoff_t > > > > > pgoff, > > > > > + const struct file *file, void **vm_private_data); > > > > > /* Called any time before splitting to check if it's allowed */ > > > > > int (*may_split)(struct vm_area_struct *vma, unsigned long addr); > > > > > int (*mremap)(struct vm_area_struct *vma); > > > > > diff --git a/mm/internal.h b/mm/internal.h > > > > > index 7bfa85b5e78b..f0f2cf1caa36 100644 > > > > > --- a/mm/internal.h > > > > > +++ b/mm/internal.h > > > > > @@ -158,6 +158,8 @@ static inline void *folio_raw_mapping(const > > > > > struct folio *folio) > > > > > * mmap hook and safely handle error conditions. On error, VMA hooks > > > > > will be > > > > > * mutated. > > > > > * > > > > > + * IMPORTANT: f_op->mmap() is deprecated, prefer > > > > > f_op->mmap_prepare(). > > > > > + * > > > > What exactly would one do to "prefer f_op->mmap_prepare()"? > > I'm saying a person should implement f_op->mmap_prepare() rather than > f_op->mmap(), since the latter is deprecated :) > > I think that's pretty clear no? > > > Since you are adding this comment for mmap_file(), I think you need to > > describe more specifically what one should call instead. > > I think it'd be a complete distraction, since if you're at the point of > calling > mmap_file() you're already not implement mmap_prepare except as a compatbility > layer.
Yep, it seems like a warning that comes too late. > > I mean maybe I'll just drop this as it seems to be causing confusion. Maybe instead we add a comment that f_ops->mmap is deprecated in favor of f_ops->mmap_prepare() in here: https://elixir.bootlin.com/linux/v7.0-rc4/source/include/linux/fs.h#L1940 ? > > > > > > > > * @file: File which backs the mapping. > > > > > * @vma: VMA which we are mapping. > > > > > * > > > > > @@ -201,6 +203,14 @@ static inline void vma_close(struct > > > > > vm_area_struct *vma) > > > > > /* unmap_vmas is in mm/memory.c */ > > > > > void unmap_vmas(struct mmu_gather *tlb, struct unmap_desc *unmap); > > > > > > > > > > +static inline void unmap_vma_locked(struct vm_area_struct *vma) > > > > > +{ > > > > > + const size_t len = vma_pages(vma) << PAGE_SHIFT; > > > > > + > > > > > + mmap_assert_locked(vma->vm_mm); > > > > You must hold the mmap write lock when unmapping. Would be better to > > assert mmap_assert_write_locked() or even vma_assert_write_locked(), > > which implies mmap_assert_write_locked(). > > I'm not sure why we don't assert this in those paths. > > I think I assumed we could only assert readonly because one of those paths > downgrades the mmap write lock to a read lock. > > I don't think we can do a VMA write lock assert here, since at the point of > do_munmap() all callers can't possibly have the VMA write lock, since they are > _looking up_ the VMA at the specified address. It sounds strange to me that we are unmapping a VMA that was not locked beforehand. Let me look into the call chains a bit more to convince myself one way or the other. The fact that do_munmap() looks up the VMA by address and then write-locks it inside vms_gather_munmap_vmas() does not mean the VMA was not already locked. vma_start_write() is re-entrant. > > But I can convert this to an mmap_assert_write_locked()! Ok, let's go with that. I don't want to slow down your patchset while I investigate locking rules here. We can strengthen the assertion later. > > > > > > > > + do_munmap(vma->vm_mm, vma->vm_start, len, NULL); > > > > > +} > > > > > + > > > > > #ifdef CONFIG_MMU > > > > > > > > > > static inline void get_anon_vma(struct anon_vma *anon_vma) > > > > > diff --git a/mm/util.c b/mm/util.c > > > > > index dba1191725b6..2b0ed54008d6 100644 > > > > > --- a/mm/util.c > > > > > +++ b/mm/util.c > > > > > @@ -1163,6 +1163,55 @@ void flush_dcache_folio(struct folio *folio) > > > > > EXPORT_SYMBOL(flush_dcache_folio); > > > > > #endif > > > > > > > > > > +static int __compat_vma_mmap(struct file *file, struct > > > > > vm_area_struct *vma) > > > > > +{ > > > > > + struct vm_area_desc desc = { > > > > > + .mm = vma->vm_mm, > > > > > + .file = file, > > > > > + .start = vma->vm_start, > > > > > + .end = vma->vm_end, > > > > > + > > > > > + .pgoff = vma->vm_pgoff, > > > > > + .vm_file = vma->vm_file, > > > > > + .vma_flags = vma->flags, > > > > > + .page_prot = vma->vm_page_prot, > > > > > + > > > > > + .action.type = MMAP_NOTHING, /* Default */ > > > > > + }; > > > > > + int err; > > > > > + > > > > > + err = vfs_mmap_prepare(file, &desc); > > > > > + if (err) > > > > > + return err; > > > > > + > > > > > + err = mmap_action_prepare(&desc, &desc.action); > > > > > + if (err) > > > > > + return err; > > > > > + > > > > > + set_vma_from_desc(vma, &desc); > > > > > + return mmap_action_complete(vma, &desc.action); > > > > > +} > > > > > + > > > > > +static int __compat_vma_mapped(struct file *file, struct > > > > > vm_area_struct *vma) > > > > > +{ > > > > > + const struct vm_operations_struct *vm_ops = vma->vm_ops; > > > > > + void *vm_private_data = vma->vm_private_data; > > > > > + int err; > > > > > + > > > > > + if (!vm_ops->mapped) > > > > > + return 0; > > > > > + > > > > > > > > Hello! > > > > > > > > Can vm_ops be NULL here? __compat_vma_mapped() is called from > > > > compat_vma_mmap(), which is reached when a filesystem provides > > > > mmap_prepare. If the mmap_prepare hook does not set desc->vm_ops, > > > > vma->vm_ops will be NULL and this dereferences a NULL pointer. > > > > > > I _think_ for this to ever be invoked, you would need to be dealing with a > > > file-backed VMA so vm_ops->fault would HAVE to be defined. > > > > > > But you're right anyway as a matter of principle we should check it! Will > > > fix. > > > > > > > > > > > For e.g. drivers/char/mem.c, mmap_zero_prepare() would trigger > > > > a NULL pointer dereference here. > > > > > > > > Would need to do > > > > if (!vm_ops || !vm_ops->mapped) > > > > return 0; > > > > > > > > here > > > > > > Yes. > > > > > > > > > > > > > > > > + err = vm_ops->mapped(vma->vm_start, vma->vm_end, vma->vm_pgoff, > > > > > file, > > > > > + &vm_private_data); > > > > > + if (err) > > > > > + unmap_vma_locked(vma); > > > > > > > > when mapped() returns an error, unmap_vma_locked(vma) is called > > > > but execution continues into the vm_private_data update below. After > > > > unmap_vma_locked() the VMA may be freed (do_munmap can remove the VMA > > > > entirely), so accessing vma->vm_private_data after that is a > > > > use-after-free. > > > > > > Very good point :) will fix thanks! > > > > > > Probably: > > > > > > if (err) > > > unmap_vma_locked(vma); > > > else if (vm_private_data != vma->vm_private_data) > > > vma->vm_private_data = vm_private_data; > > > > > > return err; > > > > > > Would be fine. > > > > > > > > > > > Probably need to do: > > > > if (err) { > > > > unmap_vma_locked(vma); > > > > return err; > > > > } > > > > > > > > > + /* Update private data if changed. */ > > > > > + if (vm_private_data != vma->vm_private_data) > > > > > + vma->vm_private_data = vm_private_data; > > > > > + > > > > > + return err; > > > > > +} > > > > > + > > > > > /** > > > > > * compat_vma_mmap() - Apply the file's .mmap_prepare() hook to an > > > > > * existing VMA and execute any requested actions. > > > > > @@ -1191,34 +1240,26 @@ EXPORT_SYMBOL(flush_dcache_folio); > > > > > */ > > > > > int compat_vma_mmap(struct file *file, struct vm_area_struct *vma) > > > > > { > > > > > - struct vm_area_desc desc = { > > > > > - .mm = vma->vm_mm, > > > > > - .file = file, > > > > > - .start = vma->vm_start, > > > > > - .end = vma->vm_end, > > > > > - > > > > > - .pgoff = vma->vm_pgoff, > > > > > - .vm_file = vma->vm_file, > > > > > - .vma_flags = vma->flags, > > > > > - .page_prot = vma->vm_page_prot, > > > > > - > > > > > - .action.type = MMAP_NOTHING, /* Default */ > > > > > - }; > > > > > int err; > > > > > > > > > > - err = vfs_mmap_prepare(file, &desc); > > > > > - if (err) > > > > > - return err; > > > > > - > > > > > - err = mmap_action_prepare(&desc, &desc.action); > > > > > + err = __compat_vma_mmap(file, vma); > > > > > if (err) > > > > > return err; > > > > > > > > > > - set_vma_from_desc(vma, &desc); > > > > > - return mmap_action_complete(vma, &desc.action); > > > > > + return __compat_vma_mapped(file, vma); > > > > > } > > > > > EXPORT_SYMBOL(compat_vma_mmap); > > > > > > > > > > +int __vma_check_mmap_hook(struct vm_area_struct *vma) > > > > > +{ > > > > > + /* vm_ops->mapped is not valid if mmap() is specified. */ > > > > > + if (WARN_ON_ONCE(vma->vm_ops->mapped)) > > > > > + return -EINVAL; > > > > > > > > I think vma->vm_ops can be NULL here. Should be: > > > > > > > > if (vma->vm_ops && WARN_ON_ONCE(vma->vm_ops->mapped)) > > > > return -EINVAL; > > > > > > I think again you'd probably only invoke this on file-backed so be ok, > > > but again > > > as a matter of principle we should check it so will fix, thanks! > > > > > > > > > > > > + > > > > > + return 0; > > > > > +} > > > > > +EXPORT_SYMBOL(__vma_check_mmap_hook); > > > > nit: Any reason __vma_check_mmap_hook() is not inlined next to its > > user vfs_mmap()? > > Headers fun, fs.h is a 'before mm.h' header, so vm_operations_struct is not > declared yet here, so we can't actually do the check there. Ack. > > > > > > > > + > > > > > static void set_ps_flags(struct page_snapshot *ps, const struct > > > > > folio *folio, > > > > > const struct page *page) > > > > > { > > > > > @@ -1316,10 +1357,7 @@ static int mmap_action_finish(struct > > > > > vm_area_struct *vma, > > > > > * invoked if we do NOT merge, so we only clean up the VMA we > > > > > created. > > > > > */ > > > > > if (err) { > > > > > - const size_t len = vma_pages(vma) << PAGE_SHIFT; > > > > > - > > > > > - do_munmap(current->mm, vma->vm_start, len, NULL); > > > > > - > > > > > + unmap_vma_locked(vma); > > > > > if (action->error_hook) { > > > > > /* We may want to filter the error. */ > > > > > err = action->error_hook(err); > > > > > diff --git a/mm/vma.c b/mm/vma.c > > > > > index 054cf1d262fb..ef9f5a5365d1 100644 > > > > > --- a/mm/vma.c > > > > > +++ b/mm/vma.c > > > > > @@ -2705,21 +2705,35 @@ static bool can_set_ksm_flags_early(struct > > > > > mmap_state *map) > > > > > return false; > > > > > } > > > > > > > > > > -static int call_action_complete(struct mmap_state *map, > > > > > - struct mmap_action *action, > > > > > - struct vm_area_struct *vma) > > > > > +static int call_mapped_hook(struct vm_area_struct *vma) > > > > > { > > > > > - int ret; > > > > > + const struct vm_operations_struct *vm_ops = vma->vm_ops; > > > > > + void *vm_private_data = vma->vm_private_data; > > > > > + int err; > > > > > > > > > > - ret = mmap_action_complete(vma, action); > > > > > + if (!vm_ops || !vm_ops->mapped) > > > > > + return 0; > > > > > + err = vm_ops->mapped(vma->vm_start, vma->vm_end, vma->vm_pgoff, > > > > > + vma->vm_file, &vm_private_data); > > > > > + if (err) { > > > > > + unmap_vma_locked(vma); > > > > > + return err; > > > > > + } > > > > > + /* Update private data if changed. */ > > > > > + if (vm_private_data != vma->vm_private_data) > > > > > + vma->vm_private_data = vm_private_data; > > > > > + return 0; > > > > > +} > > > > > > > > > > - /* If we held the file rmap we need to release it. */ > > > > > - if (map->hold_file_rmap_lock) { > > > > > - struct file *file = vma->vm_file; > > > > > +static void maybe_drop_file_rmap_lock(struct mmap_state *map, > > > > > + struct vm_area_struct *vma) > > > > > +{ > > > > > + struct file *file; > > > > > > > > > > - i_mmap_unlock_write(file->f_mapping); > > > > > - } > > > > > - return ret; > > > > > + if (!map->hold_file_rmap_lock) > > > > > + return; > > > > > + file = vma->vm_file; > > > > > + i_mmap_unlock_write(file->f_mapping); > > > > > } > > > > > > > > > > static unsigned long __mmap_region(struct file *file, unsigned long > > > > > addr, > > > > > @@ -2773,8 +2787,11 @@ static unsigned long __mmap_region(struct file > > > > > *file, unsigned long addr, > > > > > __mmap_complete(&map, vma); > > > > > > > > > > if (have_mmap_prepare && allocated_new) { > > > > > - error = call_action_complete(&map, &desc.action, vma); > > > > > + error = mmap_action_complete(vma, &desc.action); > > > > > + if (!error) > > > > > + error = call_mapped_hook(vma); > > > > > > > > > > + maybe_drop_file_rmap_lock(&map, vma); > > > > > if (error) > > > > > return error; > > > > > } > > > > > diff --git a/tools/testing/vma/include/dup.h > > > > > b/tools/testing/vma/include/dup.h > > > > > index 908beb263307..47d8db809f31 100644 > > > > > --- a/tools/testing/vma/include/dup.h > > > > > +++ b/tools/testing/vma/include/dup.h > > > > > @@ -606,12 +606,34 @@ struct vm_area_struct { > > > > > } __randomize_layout; > > > > > > > > > > struct vm_operations_struct { > > > > > - void (*open)(struct vm_area_struct * area); > > > > > + /** > > > > > + * @open: Called when a VMA is remapped or split. Not called upon > > > > > first > > > > > + * mapping a VMA. > > > > > + * Context: User context. May sleep. Caller holds mmap_lock. > > > > > + */ > > > > This comment should have been introduced in the previous patch. > > It's the testing code, it's not really important. But if I respin I'll fix... > :) Thanks! > > > > > > > > + void (*open)(struct vm_area_struct *vma); > > > > > /** > > > > > * @close: Called when the VMA is being removed from the MM. > > > > > * Context: User context. May sleep. Caller holds mmap_lock. > > > > > */ > > > > > - void (*close)(struct vm_area_struct * area); > > > > > + void (*close)(struct vm_area_struct *vma); > > > > > + /** > > > > > + * @mapped: Called when the VMA is first mapped in the MM. Not > > > > > called if > > > > > + * the new VMA is merged with an adjacent VMA. > > > > > + * > > > > > + * The @vm_private_data field is an output field allowing the > > > > > user to > > > > > + * modify vma->vm_private_data as necessary. > > > > > + * > > > > > + * ONLY valid if set from f_op->mmap_prepare. Will result in an > > > > > error if > > > > > + * set from f_op->mmap. > > > > > + * > > > > > + * Returns %0 on success, or an error otherwise. On error, the > > > > > VMA will > > > > > + * be unmapped. > > > > > + * > > > > > + * Context: User context. May sleep. Caller holds mmap_lock. > > > > > + */ > > > > > + int (*mapped)(unsigned long start, unsigned long end, pgoff_t > > > > > pgoff, > > > > > + const struct file *file, void **vm_private_data); > > > > > /* Called any time before splitting to check if it's allowed */ > > > > > int (*may_split)(struct vm_area_struct *area, unsigned long addr); > > > > > int (*mremap)(struct vm_area_struct *area); > > > > > @@ -1345,3 +1367,11 @@ static inline void vma_set_file(struct > > > > > vm_area_struct *vma, struct file *file) > > > > > swap(vma->vm_file, file); > > > > > fput(file); > > > > > } > > > > > + > > > > > +static inline void unmap_vma_locked(struct vm_area_struct *vma) > > > > > +{ > > > > > + const size_t len = vma_pages(vma) << PAGE_SHIFT; > > > > > + > > > > > + mmap_assert_locked(vma->vm_mm); > > > > > + do_munmap(vma->vm_mm, vma->vm_start, len, NULL); > > > > > +} > > > > > -- > > > > > 2.53.0 > > > > > > > > > > > > > > > > Cheers, Lorenzo
