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Subject[PATCH v2 3/7] mm/thp: fix vma_address() if virtual address below file offset
Running certain tests with a DEBUG_VM kernel would crash within hours,
on the total_mapcount BUG() in split_huge_page_to_list(), while trying
to free up some memory by punching a hole in a shmem huge page: split's
try_to_unmap() was unable to find all the mappings of the page (which,
on a !DEBUG_VM kernel, would then keep the huge page pinned in memory).

When that BUG() was changed to a WARN(), it would later crash on the
VM_BUG_ON_VMA(end < vma->vm_start || start >= vma->vm_end, vma) in
mm/internal.h:vma_address(), used by rmap_walk_file() for try_to_unmap().

vma_address() is usually correct, but there's a wraparound case when the
vm_start address is unusually low, but vm_pgoff not so low: vma_address()
chooses max(start, vma->vm_start), but that decides on the wrong address,
because start has become almost ULONG_MAX.

Rewrite vma_address() to be more careful about vm_pgoff; move the
VM_BUG_ON_VMA() out of it, returning -EFAULT for errors, so that it can
be safely used from page_mapped_in_vma() and page_address_in_vma() too.

Add vma_address_end() to apply similar care to end address calculation,
in page_vma_mapped_walk() and page_mkclean_one() and try_to_unmap_one();
though it raises a question of whether callers would do better to supply
pvmw->end to page_vma_mapped_walk() - I chose not, for a smaller patch.

An irritation is that their apparent generality breaks down on KSM pages,
which cannot be located by the page->index that page_to_pgoff() uses: as
4b0ece6fa016 ("mm: migrate: fix remove_migration_pte() for ksm pages")
once discovered. I dithered over the best thing to do about that, and
have ended up with a VM_BUG_ON_PAGE(PageKsm) in both vma_address() and
vma_address_end(); though the only place in danger of using it on them
was try_to_unmap_one().

Sidenote: vma_address() and vma_address_end() now use compound_nr() on
a head page, instead of thp_size(): to make the right calculation on a
hugetlbfs page, whether or not THPs are configured. try_to_unmap() is
used on hugetlbfs pages, but perhaps the wrong calculation never mattered.

Fixes: a8fa41ad2f6f ("mm, rmap: check all VMAs that PTE-mapped THP can be part of")
Signed-off-by: Hugh Dickins <hughd@google.com>
Cc: <stable@vger.kernel.org>
---
v2: use compound_nr() as Matthew suggested.

mm/internal.h | 50 +++++++++++++++++++++++++++++++------------
mm/page_vma_mapped.c | 16 +++++--------
mm/rmap.c | 16 ++++++-------
3 files changed, 51 insertions(+), 31 deletions(-)

--- a/mm/internal.h
+++ b/mm/internal.h
@@ -384,27 +384,51 @@ static inline void mlock_migrate_page(st
extern pmd_t maybe_pmd_mkwrite(pmd_t pmd, struct vm_area_struct *vma);

/*
- * At what user virtual address is page expected in @vma?
+ * At what user virtual address is page expected in vma?
+ * Returns -EFAULT if all of the page is outside the range of vma.
+ * If page is a compound head, the entire compound page is considered.
*/
static inline unsigned long
-__vma_address(struct page *page, struct vm_area_struct *vma)
+vma_address(struct page *page, struct vm_area_struct *vma)
{
- pgoff_t pgoff = page_to_pgoff(page);
- return vma->vm_start + ((pgoff - vma->vm_pgoff) << PAGE_SHIFT);
+ pgoff_t pgoff;
+ unsigned long address;
+
+ VM_BUG_ON_PAGE(PageKsm(page), page); /* KSM page->index unusable */
+ pgoff = page_to_pgoff(page);
+ if (pgoff >= vma->vm_pgoff) {
+ address = vma->vm_start +
+ ((pgoff - vma->vm_pgoff) << PAGE_SHIFT);
+ /* Check for address beyond vma (or wrapped through 0?) */
+ if (address < vma->vm_start || address >= vma->vm_end)
+ address = -EFAULT;
+ } else if (PageHead(page) &&
+ pgoff + compound_nr(page) > vma->vm_pgoff) {
+ address = vma->vm_start;
+ } else {
+ address = -EFAULT;
+ }
+ return address;
}

+/*
+ * Then at what user virtual address will none of the page be found in vma?
+ * Assumes that vma_address() already returned a good starting address.
+ * If page is a compound head, the entire compound page is considered.
+ */
static inline unsigned long
-vma_address(struct page *page, struct vm_area_struct *vma)
+vma_address_end(struct page *page, struct vm_area_struct *vma)
{
- unsigned long start, end;
-
- start = __vma_address(page, vma);
- end = start + thp_size(page) - PAGE_SIZE;
-
- /* page should be within @vma mapping range */
- VM_BUG_ON_VMA(end < vma->vm_start || start >= vma->vm_end, vma);
+ pgoff_t pgoff;
+ unsigned long address;

- return max(start, vma->vm_start);
+ VM_BUG_ON_PAGE(PageKsm(page), page); /* KSM page->index unusable */
+ pgoff = page_to_pgoff(page) + compound_nr(page);
+ address = vma->vm_start + ((pgoff - vma->vm_pgoff) << PAGE_SHIFT);
+ /* Check for address beyond vma (or wrapped through 0?) */
+ if (address < vma->vm_start || address > vma->vm_end)
+ address = vma->vm_end;
+ return address;
}

static inline struct file *maybe_unlock_mmap_for_io(struct vm_fault *vmf,
--- a/mm/page_vma_mapped.c
+++ b/mm/page_vma_mapped.c
@@ -225,18 +225,18 @@ restart:
if (!map_pte(pvmw))
goto next_pte;
while (1) {
+ unsigned long end;
+
if (check_pte(pvmw))
return true;
next_pte:
/* Seek to next pte only makes sense for THP */
if (!PageTransHuge(pvmw->page) || PageHuge(pvmw->page))
return not_found(pvmw);
+ end = vma_address_end(pvmw->page, pvmw->vma);
do {
pvmw->address += PAGE_SIZE;
- if (pvmw->address >= pvmw->vma->vm_end ||
- pvmw->address >=
- __vma_address(pvmw->page, pvmw->vma) +
- thp_size(pvmw->page))
+ if (pvmw->address >= end)
return not_found(pvmw);
/* Did we cross page table boundary? */
if (pvmw->address % PMD_SIZE == 0) {
@@ -274,14 +274,10 @@ int page_mapped_in_vma(struct page *page
.vma = vma,
.flags = PVMW_SYNC,
};
- unsigned long start, end;
-
- start = __vma_address(page, vma);
- end = start + thp_size(page) - PAGE_SIZE;

- if (unlikely(end < vma->vm_start || start >= vma->vm_end))
+ pvmw.address = vma_address(page, vma);
+ if (pvmw.address == -EFAULT)
return 0;
- pvmw.address = max(start, vma->vm_start);
if (!page_vma_mapped_walk(&pvmw))
return 0;
page_vma_mapped_walk_done(&pvmw);
--- a/mm/rmap.c
+++ b/mm/rmap.c
@@ -707,7 +707,6 @@ static bool should_defer_flush(struct mm
*/
unsigned long page_address_in_vma(struct page *page, struct vm_area_struct *vma)
{
- unsigned long address;
if (PageAnon(page)) {
struct anon_vma *page__anon_vma = page_anon_vma(page);
/*
@@ -722,10 +721,8 @@ unsigned long page_address_in_vma(struct
return -EFAULT;
} else
return -EFAULT;
- address = __vma_address(page, vma);
- if (unlikely(address < vma->vm_start || address >= vma->vm_end))
- return -EFAULT;
- return address;
+
+ return vma_address(page, vma);
}

pmd_t *mm_find_pmd(struct mm_struct *mm, unsigned long address)
@@ -919,7 +916,7 @@ static bool page_mkclean_one(struct page
*/
mmu_notifier_range_init(&range, MMU_NOTIFY_PROTECTION_PAGE,
0, vma, vma->vm_mm, address,
- min(vma->vm_end, address + page_size(page)));
+ vma_address_end(page, vma));
mmu_notifier_invalidate_range_start(&range);

while (page_vma_mapped_walk(&pvmw)) {
@@ -1435,9 +1432,10 @@ static bool try_to_unmap_one(struct page
* Note that the page can not be free in this function as call of
* try_to_unmap() must hold a reference on the page.
*/
+ range.end = PageKsm(page) ?
+ address + PAGE_SIZE : vma_address_end(page, vma);
mmu_notifier_range_init(&range, MMU_NOTIFY_CLEAR, 0, vma, vma->vm_mm,
- address,
- min(vma->vm_end, address + page_size(page)));
+ address, range.end);
if (PageHuge(page)) {
/*
* If sharing is possible, start and end will be adjusted
@@ -1889,6 +1887,7 @@ static void rmap_walk_anon(struct page *
struct vm_area_struct *vma = avc->vma;
unsigned long address = vma_address(page, vma);

+ VM_BUG_ON_VMA(address == -EFAULT, vma);
cond_resched();

if (rwc->invalid_vma && rwc->invalid_vma(vma, rwc->arg))
@@ -1943,6 +1942,7 @@ static void rmap_walk_file(struct page *
pgoff_start, pgoff_end) {
unsigned long address = vma_address(page, vma);

+ VM_BUG_ON_VMA(address == -EFAULT, vma);
cond_resched();

if (rwc->invalid_vma && rwc->invalid_vma(vma, rwc->arg))
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