From: Klara Modin <klarasmodin@gmail.com>
To: Lance Yang <lance.yang@linux.dev>
Cc: david@kernel.org, richard.weiyang@gmail.com,
akpm@linux-foundation.org, ljs@kernel.org, riel@surriel.com,
liam@infradead.org, vbabka@kernel.org, harry@kernel.org,
jannh@google.com, balbirs@nvidia.com, sj@kernel.org,
ziy@nvidia.com, linux-mm@kvack.org,
linux-kernel@vger.kernel.org, stable@vger.kernel.org
Subject: Re: [Patch mm-hotfixes v5] mm/page_vma_mapped: fix device-private PMD handling
Date: Wed, 1 Jul 2026 18:46:27 +0200 [thread overview]
Message-ID: <akVDNLGaCfr-PF8K@soda.int.kasm.eu> (raw)
In-Reply-To: <20260701163356.22936-1-lance.yang@linux.dev>
On 2026-07-02 00:33:56 +0800, Lance Yang wrote:
>
> On Wed, Jul 01, 2026 at 05:36:33PM +0200, David Hildenbrand (Arm) wrote:
> >On 7/1/26 16:33, Klara Modin wrote:
> >> Hi,
>
> Hi,
>
> [...]
> >>
> >> This results in a build bug for my Raspberry Pi 1:
>
> Thanks for reporting this!
>
> >> In file included from <command-line>:
> >> In function ‘check_pmd’,
> >> inlined from ‘page_vma_mapped_walk’ at /home/klara/git/linux/trees/bisect/mm/page_vma_mapped.c:256:10:
> >> /home/klara/git/linux/trees/bisect/include/linux/compiler_types.h:702:45: error: call to ‘__compiletime_assert_433’ declared with attribute error: BUILD_BUG failed
> >> 702 | _compiletime_assert(condition, msg, __compiletime_assert_, __COUNTER__)
> >> | ^
> >> /home/klara/git/linux/trees/bisect/include/linux/compiler_types.h:683:25: note: in definition of macro ‘__compiletime_assert’
> >> 683 | prefix ## suffix(); \
> >> | ^~~~~~
> >> /home/klara/git/linux/trees/bisect/include/linux/compiler_types.h:702:9: note: in expansion of macro ‘_compiletime_assert’
> >> 702 | _compiletime_assert(condition, msg, __compiletime_assert_, __COUNTER__)
> >> | ^~~~~~~~~~~~~~~~~~~
> >> /home/klara/git/linux/trees/bisect/include/linux/build_bug.h:40:37: note: in expansion of macro ‘compiletime_assert’
> >> 40 | #define BUILD_BUG_ON_MSG(cond, msg) compiletime_assert(!(cond), msg)
> >> | ^~~~~~~~~~~~~~~~~~
> >> /home/klara/git/linux/trees/bisect/include/linux/build_bug.h:60:21: note: in expansion of macro ‘BUILD_BUG_ON_MSG’
> >> 60 | #define BUILD_BUG() BUILD_BUG_ON_MSG(1, "BUILD_BUG failed")
> >> | ^~~~~~~~~~~~~~~~
> >> /home/klara/git/linux/trees/bisect/include/linux/huge_mm.h:113:28: note: in expansion of macro ‘BUILD_BUG’
> >> 113 | #define HPAGE_PMD_SHIFT ({ BUILD_BUG(); 0; })
> >> | ^~~~~~~~~
> >> /home/klara/git/linux/trees/bisect/include/linux/huge_mm.h:117:26: note: in expansion of macro ‘HPAGE_PMD_SHIFT’
> >> 117 | #define HPAGE_PMD_ORDER (HPAGE_PMD_SHIFT-PAGE_SHIFT)
> >> | ^~~~~~~~~~~~~~~
> >> /home/klara/git/linux/trees/bisect/include/linux/huge_mm.h:118:26: note: in expansion of macro ‘HPAGE_PMD_ORDER’
> >> 118 | #define HPAGE_PMD_NR (1<<HPAGE_PMD_ORDER)
> >> | ^~~~~~~~~~~~~~~
> >> /home/klara/git/linux/trees/bisect/mm/page_vma_mapped.c:142:20: note: in expansion of macro ‘HPAGE_PMD_NR’
> >> 142 | if ((pfn + HPAGE_PMD_NR - 1) < pvmw->pfn)
> >> | ^~~~~~~~~~~~
> >>
> >> bisect log:
> >>
> >> # bad: [be5c93fa674f0fc3c8f359c2143abce6bbb422e6] Add linux-next specific files for 20260630
> >> git bisect start 'HEAD'
> >> # status: waiting for 'good' commit(s), 'bad' commit known
> >> # good: [dc59e4fea9d83f03bad6bddf3fa2e52491777482] Linux 7.2-rc1
> >> git bisect good dc59e4fea9d83f03bad6bddf3fa2e52491777482
> >> # bad: [6148219e90732fd06f5d7a498bda974e6a43ab4b] Merge branch 'nand/next' of https://git.kernel.org/pub/scm/linux/kernel/git/mtd/linux.git
> >> git bisect bad 6148219e90732fd06f5d7a498bda974e6a43ab4b
> >> # bad: [e0326ebe10191447ab8fa2e904080df7b743765e] Merge branch 'for-next' of https://git.kernel.org/pub/scm/linux/kernel/git/soc/soc.git
> >> git bisect bad e0326ebe10191447ab8fa2e904080df7b743765e
> >> # bad: [fbc9c5ac47cef5a2b04aef30c8e990b32dcf2548] Merge branch 'hwmon' of https://git.kernel.org/pub/scm/linux/kernel/git/groeck/linux-staging.git
> >> git bisect bad fbc9c5ac47cef5a2b04aef30c8e990b32dcf2548
> >> # bad: [e488171f6f6df6fc899a355079665fdb3c50b0e3] Merge branch 'for-linus' of https://git.kernel.org/pub/scm/linux/kernel/git/broonie/sound.git
> >> git bisect bad e488171f6f6df6fc899a355079665fdb3c50b0e3
> >> # bad: [60db0fcb8fc9d80ac0b63041c632b41a311a45f1] Merge branch 'fs-current' of linux-next
> >> git bisect bad 60db0fcb8fc9d80ac0b63041c632b41a311a45f1
> >> # good: [51021d260d682aa17b3533848a99160ab83e0c93] Merge branch 'vfs.fixes' of https://git.kernel.org/pub/scm/linux/kernel/git/vfs/vfs.git
> >> git bisect good 51021d260d682aa17b3533848a99160ab83e0c93
> >> # good: [ded56474db6552260786a65898322464b72c7540] mm: a second pagecache maintainer
> >> git bisect good ded56474db6552260786a65898322464b72c7540
> >> # good: [6c893b948351d42cfc3761cc746ab5b3d03ee7f3] Merge branch 'misc-7.2' into next-fixes
> >> git bisect good 6c893b948351d42cfc3761cc746ab5b3d03ee7f3
> >> # good: [bfcc55a14179495b0c41408908fd7b9d7785c694] lib: test_hmm: use device devt for coherent device range selection
> >> git bisect good bfcc55a14179495b0c41408908fd7b9d7785c694
> >> # good: [a27318567c92ba5482906d047e71a7aa4fd01889] Merge branch 'fixes' of https://git.kernel.org/pub/scm/linux/kernel/git/viro/vfs.git
> >> git bisect good a27318567c92ba5482906d047e71a7aa4fd01889
> >> # bad: [6887a39652cdfd4cfd3b0962662c9cbc26ce5252] mm/page_vma_mapped: fix device-private PMD handling
> >> git bisect bad 6887a39652cdfd4cfd3b0962662c9cbc26ce5252
> >> # good: [2cc6bd0efc264b9ac760c2bc74dff4f521a680a1] MAINTAINERS: s/SeongJae/SJ/
> >> git bisect good 2cc6bd0efc264b9ac760c2bc74dff4f521a680a1
> >> # first 'bad' commit: [6887a39652cdfd4cfd3b0962662c9cbc26ce5252] mm/page_vma_mapped: fix device-private PMD handling
> >>
> >>>
> >>> Fixes: 65edfda6f3f2 ("mm/rmap: extend rmap and migration support device-private entries")
> >>> Cc: <stable@vger.kernel.org>
> >>> Signed-off-by: Wei Yang <richard.weiyang@gmail.com>
> >>> Suggested-by: David Hildenbrand <david@kernel.org>
> >>> Cc: David Hildenbrand <david@kernel.org>
> >>> Cc: Balbir Singh <balbirs@nvidia.com>
> >>> Cc: SeongJae Park <sj@kernel.org>
> >>> Cc: Zi Yan <ziy@nvidia.com>
> >>> Cc: Lorenzo Stoakes <ljs@kernel.org>
> >>> Cc: Lance Yang <lance.yang@linux.dev>
> >>>
> >>> ---
> >>> v5:
> >>> * put device-private pmd handling along with the other two cases
> >>> * remove thp_migration_supported()
> >>> v4: https://lore.kernel.org/all/20260624065353.1622-1-richard.weiyang@gmail.com/T/#u
> >>> * refine subject and commit log based on Lorenzo's suggestion
> >>> * put pmd device-private entry handling in its own if branch,
> >>> suggested by Lorenzo
> >>>
> >>> v3:
> >>> * remove cleanup part, only fix the issue for device-private entry
> >>> * refine user effect description based on Lorenzo's suggestion
> >>>
> >>> v2: https://lore.kernel.org/all/20260616063436.20455-1-richard.weiyang@gmail.com/T/#u
> >>> * specify the possible error case of current code and user visible effect
> >>> * besides fix, cleanup the pmd entry handling based on David's suggestion
> >>>
> >>> v1: https://lore.kernel.org/linux-mm/20260508013728.21285-1-richard.weiyang@gmail.com/
> >>> ---
> >>> mm/page_vma_mapped.c | 30 ++++++++++++++++--------------
> >>> 1 file changed, 16 insertions(+), 14 deletions(-)
> >>>
> >>> diff --git a/mm/page_vma_mapped.c b/mm/page_vma_mapped.c
> >>> index 2ccbabfb2cc1..2d6c58488e3a 100644
> >>> --- a/mm/page_vma_mapped.c
> >>> +++ b/mm/page_vma_mapped.c
> >>> @@ -243,21 +243,30 @@ bool page_vma_mapped_walk(struct page_vma_mapped_walk *pvmw)
> >>> */
> >>> pmde = pmdp_get_lockless(pvmw->pmd);
> >>>
> >>> - if (pmd_trans_huge(pmde) || pmd_is_migration_entry(pmde)) {
> >>> + if (pmd_trans_huge(pmde) || pmd_is_migration_entry(pmde) ||
> >>> + pmd_is_device_private_entry(pmde)) {
> >>> pvmw->ptl = pmd_lock(mm, pvmw->pmd);
> >>> pmde = *pvmw->pmd;
> >>> - if (!pmd_present(pmde)) {
> >>> + if (pmd_is_migration_entry(pmde)) {
> >>> softleaf_t entry;
> >>>
> >>> - if (!thp_migration_supported() ||
> >>> - !(pvmw->flags & PVMW_MIGRATION))
> >>> + if (!(pvmw->flags & PVMW_MIGRATION))
> >>> return not_found(pvmw);
> >>> entry = softleaf_from_pmd(pmde);
> >>> + if (!check_pmd(softleaf_to_pfn(entry), pvmw))
> >>> + return not_found(pvmw);
> >>> + return true;
> >>> + } else if (pmd_is_device_private_entry(pmde)) {
> >>> + softleaf_t entry;
> >>>
> >>
> >>> - if (!softleaf_is_migration(entry) ||
> >>> - !check_pmd(softleaf_to_pfn(entry), pvmw))
> >>
> >> My only guess here would be that the compiler evaluates
> >> !softleaf_is_migration(entry) to always be true and optimises away the
> >> !check_pmd(softleaf_to_pfn(entry), pvmw) which is why this worked
> >> before?
> >
> >Weird, we enter this path only with
> >
> >pmd_trans_huge(pmde) || pmd_is_migration_entry(pmde) ||
> >pmd_is_device_private_entry(pmde)
> >
> >If any one of these would compile for !CONFIG_TRANSPARENT_HUGEPAGE that would be
> >odd.
> >
> >pmd_is_device_private_entry() is hard-coded to false unless
> >CONFIG_ARCH_ENABLE_THP_MIGRATION. Which is only selected with
> >ARCH_ENABLE_THP_MIGRATION.
> >
> >pmd_trans_huge() as well.
> >
> >Maybe it's struggling with pmd_is_migration_entry() on some (older) compilers?
> >(not innlining stuff and not properly optimizing it out).
It's a GCC 16 cross-compiler for armv6 so I wouldn't call it old :)
> >
> >The whole conditional must be optimized out.
>
> Right. Kinda weird if compiler didn't fold
>
> pmd_trans_huge(pmde) || pmd_is_migration_entry(pmde) ||
> pmd_is_device_private_entry(pmde)
>
> away here ...
>
> >We could check for IS_ENABLED(CONFIG_TRANSPARENT_HUGEPAGE)) right at the start
> >to make it easier for the compiler:
>
> +1, explicit THP guard should do the trick :)
>
> >if (IS_ENABLED(CONFIG_TRANSPARENT_HUGEPAGE)) &&
> > (pmd_trans_huge(pmde) || pmd_is_migration_entry(pmde) ||
> > pmd_is_device_private_entry(pmde))) {
> >
> >
>
> Klara, could you try with this change and see if it fixes the build?
>
> Thanks, Lance
This does indeed make it build.
Thanks,
Klara Modin
next prev parent reply other threads:[~2026-07-01 16:46 UTC|newest]
Thread overview: 8+ messages / expand[flat|nested] mbox.gz Atom feed top
2026-06-30 2:15 [Patch mm-hotfixes v5] mm/page_vma_mapped: fix device-private PMD handling Wei Yang
2026-06-30 2:43 ` Lance Yang
2026-06-30 3:57 ` Balbir Singh
2026-07-01 14:33 ` Klara Modin
2026-07-01 15:36 ` David Hildenbrand (Arm)
2026-07-01 16:33 ` Lance Yang
2026-07-01 16:46 ` Klara Modin [this message]
2026-07-02 17:49 ` Lorenzo Stoakes
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