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Subject[PATCH 5.4 64/71] mm/thp: fix page_vma_mapped_walk() if THP mapped by ptes
Date
From: Hugh Dickins <hughd@google.com>

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).

Crash dumps showed two tail pages of a shmem huge page remained mapped
by pte: ptes in a non-huge-aligned vma of a gVisor process, at the end
of a long unmapped range; and no page table had yet been allocated for
the head of the huge page to be mapped into.

Although designed to handle these odd misaligned huge-page-mapped-by-pte
cases, page_vma_mapped_walk() falls short by returning false prematurely
when !pmd_present or !pud_present or !p4d_present or !pgd_present: there
are cases when a huge page may span the boundary, with ptes present in
the next.

Restructure page_vma_mapped_walk() as a loop to continue in these cases,
while keeping its layout much as before. Add a step_forward() helper to
advance pvmw->address across those boundaries: originally I tried to use
mm's standard p?d_addr_end() macros, but hit the same crash 512 times
less often: because of the way redundant levels are folded together, but
folded differently in different configurations, it was just too
difficult to use them correctly; and step_forward() is simpler anyway.

Link: https://lkml.kernel.org/r/fedb8632-1798-de42-f39e-873551d5bc81@google.com
Fixes: ace71a19cec5 ("mm: introduce page_vma_mapped_walk()")
Signed-off-by: Hugh Dickins <hughd@google.com>
Acked-by: Kirill A. Shutemov <kirill.shutemov@linux.intel.com>
Cc: Alistair Popple <apopple@nvidia.com>
Cc: Matthew Wilcox <willy@infradead.org>
Cc: Peter Xu <peterx@redhat.com>
Cc: Ralph Campbell <rcampbell@nvidia.com>
Cc: Wang Yugui <wangyugui@e16-tech.com>
Cc: Will Deacon <will@kernel.org>
Cc: Yang Shi <shy828301@gmail.com>
Cc: Zi Yan <ziy@nvidia.com>
Cc: <stable@vger.kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
---
mm/page_vma_mapped.c | 34 +++++++++++++++++++++++++---------
1 file changed, 25 insertions(+), 9 deletions(-)

diff --git a/mm/page_vma_mapped.c b/mm/page_vma_mapped.c
index 93dd79ba9969..0e8f54683709 100644
--- a/mm/page_vma_mapped.c
+++ b/mm/page_vma_mapped.c
@@ -111,6 +111,13 @@ static bool check_pte(struct page_vma_mapped_walk *pvmw)
return pfn_in_hpage(pvmw->page, pfn);
}

+static void step_forward(struct page_vma_mapped_walk *pvmw, unsigned long size)
+{
+ pvmw->address = (pvmw->address + size) & ~(size - 1);
+ if (!pvmw->address)
+ pvmw->address = ULONG_MAX;
+}
+
/**
* page_vma_mapped_walk - check if @pvmw->page is mapped in @pvmw->vma at
* @pvmw->address
@@ -178,16 +185,22 @@ bool page_vma_mapped_walk(struct page_vma_mapped_walk *pvmw)
if (pvmw->pte)
goto next_pte;
restart:
- {
+ do {
pgd = pgd_offset(mm, pvmw->address);
- if (!pgd_present(*pgd))
- return false;
+ if (!pgd_present(*pgd)) {
+ step_forward(pvmw, PGDIR_SIZE);
+ continue;
+ }
p4d = p4d_offset(pgd, pvmw->address);
- if (!p4d_present(*p4d))
- return false;
+ if (!p4d_present(*p4d)) {
+ step_forward(pvmw, P4D_SIZE);
+ continue;
+ }
pud = pud_offset(p4d, pvmw->address);
- if (!pud_present(*pud))
- return false;
+ if (!pud_present(*pud)) {
+ step_forward(pvmw, PUD_SIZE);
+ continue;
+ }

pvmw->pmd = pmd_offset(pud, pvmw->address);
/*
@@ -234,7 +247,8 @@ bool page_vma_mapped_walk(struct page_vma_mapped_walk *pvmw)

spin_unlock(ptl);
}
- return false;
+ step_forward(pvmw, PMD_SIZE);
+ continue;
}
if (!map_pte(pvmw))
goto next_pte;
@@ -264,7 +278,9 @@ bool page_vma_mapped_walk(struct page_vma_mapped_walk *pvmw)
spin_lock(pvmw->ptl);
}
goto this_pte;
- }
+ } while (pvmw->address < end);
+
+ return false;
}

/**
--
2.30.2
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