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[2001:678:a5c:1202:7b92:9ac1:b9ef:5287]) by smtp.gmail.com with ESMTPSA id 2adb3069b0e04-5aec894e5afsm140232e87.0.2026.07.01.09.46.29 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Wed, 01 Jul 2026 09:46:29 -0700 (PDT) Date: Wed, 1 Jul 2026 18:46:27 +0200 From: Klara Modin To: Lance Yang 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 Message-ID: References: <20260701163356.22936-1-lance.yang@linux.dev> Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Type: text/plain; charset=utf-8 Content-Disposition: inline Content-Transfer-Encoding: 8bit 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 : > >> 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< >> | ^~~~~~~~~~~~~~~ > >> /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: > >>> Signed-off-by: Wei Yang > >>> Suggested-by: David Hildenbrand > >>> Cc: David Hildenbrand > >>> Cc: Balbir Singh > >>> Cc: SeongJae Park > >>> Cc: Zi Yan > >>> Cc: Lorenzo Stoakes > >>> Cc: Lance Yang > >>> > >>> --- > >>> 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