The patch titled
Subject: mm/thp: re-apply mkdirty for small pages after split
has been added to the -mm mm-unstable branch. Its filename is
mm-thp-re-apply-mkdirty-for-small-pages-after-split.patch
This patch will shortly appear at
https://git.kernel.org/pub/scm/linux/kernel/git/akpm/25-new.git/tree/patches/mm-thp-re-apply-mkdirty-for-small-pages-after-split.patch
This patch will later appear in the mm-unstable branch at
git://git.kernel.org/pub/scm/linux/kernel/git/akpm/mm
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------------------------------------------------------
From: Peter Xu <peterx@xxxxxxxxxx>
Subject: mm/thp: re-apply mkdirty for small pages after split
Date: Fri, 25 Nov 2022 13:58:57 -0500
We used to have 624a2c94f5b7 (Partly revert "mm/thp: carry over dirty bit
when thp splits on pmd") fixing the regression reported here by Anatoly
Pugachev on sparc64:
https://lore.kernel.org/r/20221021160603.GA23307@xxxxxxxxxxxx
Where we temporarily ignored the dirty bit for small pages.
Then, Hev also reported similar issue on loongarch:
(the original mail was private, but Anatoly copied the list here)
https://lore.kernel.org/r/CADxRZqxqb7f_WhMh=jweZP+ynf_JwGd-0VwbYgp4P+T0-AXosw@xxxxxxxxxxxxxx
Hev pointed out that the issue is having HW write bit set within the
pte_mkdirty() so the split pte can be written after split even if e.g.
they were shared by more than one processes, causing data corrupt.
Hev also tried to explain why loongarch set HW write bit in mkdirty:
https://lore.kernel.org/r/CAHirt9itKO_K_HPboXh5AyJtt16Zf0cD73PtHvM=na39u_ztxA@xxxxxxxxxxxxxx
One way to fix it is as what Huacai proposed here for loongarch (then we
can re-apply the dirty bit in thp split):
https://lore.kernel.org/r/20221117042532.4064448-1-chenhuacai@xxxxxxxxxxxx
We may need similar thing for sparc64, though.
For now since we've found the root cause of the dirty bit issue the
simpler solution (which won't lose the dirty bit for small) that will work
for both is we wr-protect after pte_mkdirty(), so the HW write bit can be
persistent after thp split.
Add a comment for wrprotect, so we will not mess up the ordering later.
With 624a2c94f5b7 (Partly revert "mm/thp: carry over dirty bit when thp
splits on pmd") this is not a fix anymore, but just brings back the dirty
bit for thp split safely, so we re-apply the optimization but in safe way.
Provide a Tested-by credit to Hev too (not the exact same patch but the
same outcome) for loongarch.
Link: https://lkml.kernel.org/r/20221125185857.3110155-1-peterx@xxxxxxxxxx
Signed-off-by: Peter Xu <peterx@xxxxxxxxxx>
Tested-by: Hev <r@xxxxxx> # loongarch
Cc: Anatoly Pugachev <matorola@xxxxxxxxx>
Cc: Raghavendra K T <raghavendra.kt@xxxxxxx>
Cc: Thorsten Leemhuis <regressions@xxxxxxxxxxxxx>
Cc: Mike Kravetz <mike.kravetz@xxxxxxxxxx>
Cc: "Kirill A. Shutemov" <kirill.shutemov@xxxxxxxxxxxxxxx>
Signed-off-by: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>
---
mm/huge_memory.c | 14 ++++++++------
1 file changed, 8 insertions(+), 6 deletions(-)
--- a/mm/huge_memory.c~mm-thp-re-apply-mkdirty-for-small-pages-after-split
+++ a/mm/huge_memory.c
@@ -2192,16 +2192,18 @@ static void __split_huge_pmd_locked(stru
entry = maybe_mkwrite(entry, vma);
if (anon_exclusive)
SetPageAnonExclusive(page + i);
- if (!write)
- entry = pte_wrprotect(entry);
if (!young)
entry = pte_mkold(entry);
+ /* NOTE: this may set soft-dirty too on some archs */
+ if (dirty)
+ entry = pte_mkdirty(entry);
/*
- * NOTE: we don't do pte_mkdirty when dirty==true
- * because it breaks sparc64 which can sigsegv
- * random process. Need to revisit when we figure
- * out what is special with sparc64.
+ * NOTE: this needs to happen after pte_mkdirty,
+ * because some archs (sparc64, loongarch) could
+ * set hw write bit when mkdirty.
*/
+ if (!write)
+ entry = pte_wrprotect(entry);
if (soft_dirty)
entry = pte_mksoft_dirty(entry);
if (uffd_wp)
_
Patches currently in -mm which might be from peterx@xxxxxxxxxx are
mm-migrate-fix-read-only-page-got-writable-when-recover-pte.patch
mm-always-compile-in-pte-markers.patch
mm-use-pte-markers-for-swap-errors.patch
mm-uffd-sanity-check-write-bit-for-uffd-wp-protected-ptes.patch
selftests-vm-use-memfd-for-hugepage-mmap-test.patch
mm-thp-re-apply-mkdirty-for-small-pages-after-split.patch
