From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from smtp.kernel.org (aws-us-west-2-korg-mail-alma10-1.taild15c8.ts.net [100.103.45.18]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by smtp.subspace.kernel.org (Postfix) with ESMTPS id 47CDA35F179; Thu, 2 Jul 2026 17:49:45 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.client-ip=100.103.45.18 ARC-Seal:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1783014586; cv=none; b=cxmm7au+1nmy50b6nhJLK+/GjkF2dlUVtoFhLb+NGqoCujkcYCoTw+kYkXylQWKkoEhoVBnWC8v81B4sHde3wmAD0xZK4DGKKeLEn5ydNjZ11rMbHf+tWUZIBz6Xyv8qkIaEoChdnyk0Lse3k2yjxVsBVT6IhdLh3BbEgQBINT0= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1783014586; c=relaxed/simple; bh=HulFaU5N9XffYtGHyYz+8oE2bb35HAROZbyr78/rhyQ=; h=Date:From:To:Cc:Subject:Message-ID:References:MIME-Version: Content-Type:Content-Disposition:In-Reply-To; b=u3Yoh7Udwgq20A94LZzG+/9WGEbE4TJwmR6NGNAX5jw+HAYuzldTiaxj+arKdIf/ZVd9EJY/itO0QNDsWOjRcg3O0qxSw2V3ORO+08fQy/z1Utyy1xOrJX0+EMQjAsIr7iazDf9mSxfJFfR9B0rpgoNdpHaeSveaum2LTmVj94o= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=kernel.org header.i=@kernel.org header.b=eS77nEYO; arc=none smtp.client-ip=100.103.45.18 Authentication-Results: smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=kernel.org header.i=@kernel.org header.b="eS77nEYO" Received: by smtp.kernel.org (Postfix) with ESMTPSA id 7393F1F000E9; Thu, 2 Jul 2026 17:49:41 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=kernel.org; s=k20260515; t=1783014585; bh=wuCIBX5SKHbYxpJ28hHw2u9p1C37eIECB4uw4lnxHb8=; h=Date:From:To:Cc:Subject:References:In-Reply-To; b=eS77nEYOKAx15MNcfxh3mmjiOoNqjHjvbu58LpEIZnqMfmdHX15eBZ0mkhikJcDl/ tP4vIFxQbFD2GzOd37ePE1DRxEY/pdgBfqyusoa3LIhU1HDeyJoKx8sECR0dXAbwj6 SKZGjK6FxJPXBg5c91F/GR6iLSBNoFSNfMZ+vbjVBwzoA24Io+ulZ2nqwXoH/j9Wnl jpNTcYvD7IihrnGPEnCBl0hbLFG1RGgapV4SsGoPbnDvBxCVSdE7OXQXGYkgVbgrsB b3caRgLyJ0IETP4mgPER75qHRXXZClI8NazztQSybbHie9vOe9eIwqTvLxI8GBAuSh VbsJeulNaTsyQ== Date: Thu, 2 Jul 2026 18:49:34 +0100 From: Lorenzo Stoakes To: Klara Modin Cc: Lance Yang , david@kernel.org, richard.weiyang@gmail.com, akpm@linux-foundation.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: On Wed, Jul 01, 2026 at 06:46:27PM +0200, Klara Modin wrote: > 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. Hmm this is pretty ugly though. softleaf_is_migration() is a bit-test so I don't think that's the issue. I think the issue here is thp_migration_supported() that's what's needed here. So maybe - if (!thp_migration_supported() || - !(pvmw->flags & PVMW_MIGRATION)) + if (!(pvmw->flags & PVMW_MIGRATION)) Should just be kept as it is or: +#ifdef CONFIG_ARCH_ENABLE_THP_MIGRATION static inline bool pmd_is_migration_entry(pmd_t pmd) { return softleaf_is_migration(softleaf_from_pmd(pmd)); } +#else +static inline bool pmd_is_migration_entry(pmd_t pmd) +{ + return false; +} +#endif And I think the CONFIG_TRANSPARENT_HUGEPAGE is just conflating the CONFIG_ARCH_ENABLE_THP_MIGRATION. > > Thanks, > Klara Modin Thanks, Lorenzo