On 02/04/2025 20:04, James Houghton wrote:
On Wed, Apr 2, 2025 at 9:07 AM Nikita Kalyazin <kalya...@amazon.com> wrote:Remove shmem-specific code from UFFDIO_CONTINUE implementation for non-huge pages by calling vm_ops->fault(). A new VMF flag, FAULT_FLAG_NO_USERFAULT_MINOR, is introduced to avoid recursive call to handle_userfault(). Signed-off-by: Nikita Kalyazin <kalya...@amazon.com> --- include/linux/mm_types.h | 3 +++ mm/hugetlb.c | 2 +- mm/shmem.c | 3 ++- mm/userfaultfd.c | 25 ++++++++++++++++++------- 4 files changed, 24 insertions(+), 9 deletions(-) diff --git a/include/linux/mm_types.h b/include/linux/mm_types.h index 0234f14f2aa6..91a00f2cd565 100644 --- a/include/linux/mm_types.h +++ b/include/linux/mm_types.h @@ -1429,6 +1429,8 @@ enum tlb_flush_reason { * @FAULT_FLAG_ORIG_PTE_VALID: whether the fault has vmf->orig_pte cached. * We should only access orig_pte if this flag set. * @FAULT_FLAG_VMA_LOCK: The fault is handled under VMA lock. + * @FAULT_FLAG_NO_USERFAULT_MINOR: The fault handler must not call userfaultfd + * minor handler.Perhaps instead a flag that says to avoid the userfaultfd minor fault handler, maybe we should have a flag to indicate that vm_ops->fault() has been called by UFFDIO_CONTINUE. See below.* * About @FAULT_FLAG_ALLOW_RETRY and @FAULT_FLAG_TRIED: we can specify * whether we would allow page faults to retry by specifying these two @@ -1467,6 +1469,7 @@ enum fault_flag { FAULT_FLAG_UNSHARE = 1 << 10, FAULT_FLAG_ORIG_PTE_VALID = 1 << 11, FAULT_FLAG_VMA_LOCK = 1 << 12, + FAULT_FLAG_NO_USERFAULT_MINOR = 1 << 13, }; typedef unsigned int __bitwise zap_flags_t; diff --git a/mm/hugetlb.c b/mm/hugetlb.c index 97930d44d460..ba90d48144fc 100644 --- a/mm/hugetlb.c +++ b/mm/hugetlb.c @@ -6228,7 +6228,7 @@ static vm_fault_t hugetlb_no_page(struct address_space *mapping, } /* Check for page in userfault range. */ - if (userfaultfd_minor(vma)) { + if (userfaultfd_minor(vma) && !(vmf->flags & FAULT_FLAG_NO_USERFAULT_MINOR)) { folio_unlock(folio); folio_put(folio); /* See comment in userfaultfd_missing() block above */ diff --git a/mm/shmem.c b/mm/shmem.c index 1ede0800e846..5e1911e39dec 100644 --- a/mm/shmem.c +++ b/mm/shmem.c @@ -2467,7 +2467,8 @@ static int shmem_get_folio_gfp(struct inode *inode, pgoff_t index, fault_mm = vma ? vma->vm_mm : NULL; folio = filemap_get_entry(inode->i_mapping, index); - if (folio && vma && userfaultfd_minor(vma)) { + if (folio && vma && userfaultfd_minor(vma) && + !(vmf->flags & FAULT_FLAG_NO_USERFAULT_MINOR)) { if (!xa_is_value(folio)) folio_put(folio); *fault_type = handle_userfault(vmf, VM_UFFD_MINOR); diff --git a/mm/userfaultfd.c b/mm/userfaultfd.c index d06453fa8aba..68a995216789 100644 --- a/mm/userfaultfd.c +++ b/mm/userfaultfd.c @@ -386,24 +386,35 @@ static int mfill_atomic_pte_continue(pmd_t *dst_pmd, unsigned long dst_addr, uffd_flags_t flags) { - struct inode *inode = file_inode(dst_vma->vm_file); - pgoff_t pgoff = linear_page_index(dst_vma, dst_addr); struct folio *folio; struct page *page; int ret; + struct vm_fault vmf = { + .vma = dst_vma, + .address = dst_addr, + .flags = FAULT_FLAG_WRITE | FAULT_FLAG_REMOTE | + FAULT_FLAG_NO_USERFAULT_MINOR, + .pte = NULL, + .page = NULL, + .pgoff = linear_page_index(dst_vma, dst_addr), + }; + + if (!dst_vma->vm_ops || !dst_vma->vm_ops->fault) + return -EINVAL; - ret = shmem_get_folio(inode, pgoff, 0, &folio, SGP_NOALLOC); - /* Our caller expects us to return -EFAULT if we failed to find folio */ - if (ret == -ENOENT) + ret = dst_vma->vm_ops->fault(&vmf);shmem_get_folio() was being called with SGP_NOALLOC, and now it is being called with SGP_CACHE (by shmem_fault()). This will result in a UAPI change: UFFDIO_CONTINUE for a VA without a page in the page cache should result in EFAULT, but now the page will be allocated. SGP_NOALLOC was carefully chosen[1], so I think a better way to do this will be to: 1. Have a FAULT_FLAG_USERFAULT_CONTINUE (or something) 2. In shmem_fault(), if FAULT_FLAG_USERFAULT_CONTINUE, use SGP_NOALLOC instead of SGP_CACHE (and make sure not to drop into handle_userfault(), of course)
Yes, that is very true. Thanks for pointing out. Will apply in the next version.
[1]: https://lore.kernel.org/linux-mm/20220610173812.1768919-1-axelrasmus...@google.com/+ if (ret & VM_FAULT_ERROR) { ret = -EFAULT; - if (ret) goto out; + } + + page = vmf.page; + folio = page_folio(page); if (!folio) {What if ret == VM_FAULT_RETRY? I think we should retry instead instead of returning -EFAULT.
True. I'm going to retry the vm_ops->fault() call in this case.
And I'm not sure how VM_FAULT_NOPAGE should be handled, like if we need special logic for it or not.
As far as I see, the only case it can come up in shmem is during a fault when a hole is being punched [1]. I'm thinking of returning EAGAIN to the userspace if this happens.
[1] https://elixir.bootlin.com/linux/v6.14-rc6/source/mm/shmem.c#L2670
ret = -EFAULT; goto out; } - page = folio_file_page(folio, pgoff); if (PageHWPoison(page)) { ret = -EIO; goto out_release; -- 2.47.1
