Avoid multiple CPU page faults to the same device page racing by trying
to lock the page in do_swap_page before taking an extra reference to the
page. This prevents scenarios where multiple CPU page faults each take
an extra reference to a device page, which could abort migration in
folio_migrate_mapping. With the device page being locked in
do_swap_page, the migrate_vma_* functions need to be updated to avoid
locking the fault_page argument.
Prior to this change, a livelock scenario could occur in Xe's (Intel GPU
DRM driver) SVM implementation if enough threads faulted the same device
page.
v3:
- Put page after unlocking page (Alistair)
- Warn on spliting a TPH which is fault page (Alistair)
- Warn on dst page == fault page (Alistair)
v6:
- Add more verbose comment around THP (Alistair)
v7:
- Fix migrate_device_finalize alignment (Checkpatch)
Cc: Alistair Popple <apop...@nvidia.com>
Cc: Philip Yang <philip.y...@amd.com>
Cc: Felix Kuehling <felix.kuehl...@amd.com>
Cc: Christian König <christian.koe...@amd.com>
Cc: Andrew Morton <a...@linux-foundation.org>
Suggested-by: Simona Vetter <simona.vet...@ffwll.ch>
Signed-off-by: Matthew Brost <matthew.br...@intel.com>
Reviewed-by: Alistair Popple <apop...@nvidia.com>
Tested-by: Alistair Popple <apop...@nvidia.com>
---
mm/memory.c | 13 ++++++---
mm/migrate_device.c | 64 ++++++++++++++++++++++++++++++++-------------
2 files changed, 55 insertions(+), 22 deletions(-)
diff --git a/mm/memory.c b/mm/memory.c
index b4d3d4893267..59b804f4bf3f 100644
--- a/mm/memory.c
+++ b/mm/memory.c
@@ -4348,10 +4348,15 @@ vm_fault_t do_swap_page(struct vm_fault *vmf)
* Get a page reference while we know the page can't be
* freed.
*/
- get_page(vmf->page);
- pte_unmap_unlock(vmf->pte, vmf->ptl);
- ret = vmf->page->pgmap->ops->migrate_to_ram(vmf);
- put_page(vmf->page);
+ if (trylock_page(vmf->page)) {
+ get_page(vmf->page);
+ pte_unmap_unlock(vmf->pte, vmf->ptl);
+ ret =
vmf->page->pgmap->ops->migrate_to_ram(vmf);
+ unlock_page(vmf->page);
+ put_page(vmf->page);
+ } else {
+ pte_unmap_unlock(vmf->pte, vmf->ptl);
+ }
} else if (is_hwpoison_entry(entry)) {
ret = VM_FAULT_HWPOISON;
} else if (is_pte_marker_entry(entry)) {
diff --git a/mm/migrate_device.c b/mm/migrate_device.c
index e85ed4ab6df2..a351497ced4a 100644
--- a/mm/migrate_device.c
+++ b/mm/migrate_device.c
@@ -60,6 +60,8 @@ static int migrate_vma_collect_pmd(pmd_t *pmdp,
struct mm_walk *walk)
{
struct migrate_vma *migrate = walk->private;
+ struct folio *fault_folio = migrate->fault_page ?
+ page_folio(migrate->fault_page) : NULL;
struct vm_area_struct *vma = walk->vma;
struct mm_struct *mm = vma->vm_mm;
unsigned long addr = start, unmapped = 0;
@@ -88,11 +90,16 @@ static int migrate_vma_collect_pmd(pmd_t *pmdp,
folio_get(folio);
spin_unlock(ptl);
+ /* FIXME: we don't expect THP for fault_folio */
+ if (WARN_ON_ONCE(fault_folio == folio))
+ return migrate_vma_collect_skip(start, end,
+ walk);
if (unlikely(!folio_trylock(folio)))
return migrate_vma_collect_skip(start, end,
walk);
ret = split_folio(folio);
- folio_unlock(folio);
+ if (fault_folio != folio)
+ folio_unlock(folio);
folio_put(folio);
if (ret)
return migrate_vma_collect_skip(start, end,
@@ -192,7 +199,7 @@ static int migrate_vma_collect_pmd(pmd_t *pmdp,
* optimisation to avoid walking the rmap later with
* try_to_migrate().
*/
- if (folio_trylock(folio)) {
+ if (fault_folio == folio || folio_trylock(folio)) {
bool anon_exclusive;
pte_t swp_pte;
@@ -204,7 +211,8 @@ static int migrate_vma_collect_pmd(pmd_t *pmdp,
if (folio_try_share_anon_rmap_pte(folio, page))
{
set_pte_at(mm, addr, ptep, pte);
- folio_unlock(folio);
+ if (fault_folio != folio)
+ folio_unlock(folio);
folio_put(folio);
mpfn = 0;
goto next;
@@ -363,6 +371,8 @@ static unsigned long migrate_device_unmap(unsigned long
*src_pfns,
unsigned long npages,
struct page *fault_page)
{
+ struct folio *fault_folio = fault_page ?
+ page_folio(fault_page) : NULL;
unsigned long i, restore = 0;
bool allow_drain = true;
unsigned long unmapped = 0;
@@ -427,7 +437,8 @@ static unsigned long migrate_device_unmap(unsigned long
*src_pfns,
remove_migration_ptes(folio, folio, 0);
src_pfns[i] = 0;
- folio_unlock(folio);
+ if (fault_folio != folio)
+ folio_unlock(folio);
folio_put(folio);
restore--;
}
@@ -536,6 +547,8 @@ int migrate_vma_setup(struct migrate_vma *args)
return -EINVAL;
if (args->fault_page && !is_device_private_page(args->fault_page))
return -EINVAL;
+ if (args->fault_page && !PageLocked(args->fault_page))
+ return -EINVAL;
memset(args->src, 0, sizeof(*args->src) * nr_pages);
args->cpages = 0;
@@ -799,19 +812,13 @@ void migrate_vma_pages(struct migrate_vma *migrate)
}
EXPORT_SYMBOL(migrate_vma_pages);
-/*
- * migrate_device_finalize() - complete page migration
- * @src_pfns: src_pfns returned from migrate_device_range()
- * @dst_pfns: array of pfns allocated by the driver to migrate memory to
- * @npages: number of pages in the range
- *
- * Completes migration of the page by removing special migration entries.
- * Drivers must ensure copying of page data is complete and visible to the CPU
- * before calling this.
- */
-void migrate_device_finalize(unsigned long *src_pfns,
- unsigned long *dst_pfns, unsigned long npages)
+static void __migrate_device_finalize(unsigned long *src_pfns,
+ unsigned long *dst_pfns,
+ unsigned long npages,
+ struct page *fault_page)
{
+ struct folio *fault_folio = fault_page ?
+ page_folio(fault_page) : NULL;
unsigned long i;
for (i = 0; i < npages; i++) {
@@ -824,6 +831,7 @@ void migrate_device_finalize(unsigned long *src_pfns,
if (!page) {
if (dst) {
+ WARN_ON_ONCE(fault_folio == dst);
folio_unlock(dst);
folio_put(dst);
}
@@ -834,6 +842,7 @@ void migrate_device_finalize(unsigned long *src_pfns,
if (!(src_pfns[i] & MIGRATE_PFN_MIGRATE) || !dst) {
if (dst) {
+ WARN_ON_ONCE(fault_folio == dst);
folio_unlock(dst);
folio_put(dst);
}
@@ -843,15 +852,33 @@ void migrate_device_finalize(unsigned long *src_pfns,
if (!folio_is_zone_device(dst))
folio_add_lru(dst);
remove_migration_ptes(src, dst, 0);
- folio_unlock(src);
+ if (fault_folio != src)
+ folio_unlock(src);
folio_put(src);
if (dst != src) {
+ WARN_ON_ONCE(fault_folio == dst);
folio_unlock(dst);
folio_put(dst);
}
}
}
+
+/*
+ * migrate_device_finalize() - complete page migration
+ * @src_pfns: src_pfns returned from migrate_device_range()
+ * @dst_pfns: array of pfns allocated by the driver to migrate memory to
+ * @npages: number of pages in the range
+ *
+ * Completes migration of the page by removing special migration entries.
+ * Drivers must ensure copying of page data is complete and visible to the CPU
+ * before calling this.
+ */
+void migrate_device_finalize(unsigned long *src_pfns,
+ unsigned long *dst_pfns, unsigned long npages)
+{
+ return __migrate_device_finalize(src_pfns, dst_pfns, npages, NULL);
+}
EXPORT_SYMBOL(migrate_device_finalize);
/**
@@ -867,7 +894,8 @@ EXPORT_SYMBOL(migrate_device_finalize);
*/
void migrate_vma_finalize(struct migrate_vma *migrate)
{
- migrate_device_finalize(migrate->src, migrate->dst, migrate->npages);
+ __migrate_device_finalize(migrate->src, migrate->dst, migrate->npages,
+ migrate->fault_page);
}
EXPORT_SYMBOL(migrate_vma_finalize);
--
2.34.1
