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b=Q6wQtfYON2Lo65wpxSC2KY3N3ETpsdAh0mAs3IpVAIoLlyWrSEoAGOwtAXQy/kQV2 b5kcexlK+Zc/LTP6q608vEkFO9tLtb+LmsFA7M0fqFtsfgghYyLvU0f3yK1GESb67N bs5iwV/1KijpkOlb9Sk7dgOZML1JcNDPTaxdjmph6JRGeB67If/iWuYKST0m1XGc7/ 3i8f88qIW1XWfXworlXF9yKKH1oXVujLBEM8ylGEkUw0jO1SeGhFycTkiTxqqL89ow j6Dwlx3xX6ISfWUQ+1dztTeKihqzLo0/CVuK3QpFdpHlf9F5cKFSn5lLiuuoJ0DFGU rhk+2ZqAMbljA== Message-ID: <113dddc5-27e3-4e9e-a90c-f076a4629f51@kernel.org> Date: Tue, 12 May 2026 20:55:47 +0200 MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH] mm/page_vma_mapped: revalidate and do proper check before return device-private pmd To: Wei Yang Cc: Balbir Singh , akpm@linux-foundation.org, ljs@kernel.org, riel@surriel.com, liam@infradead.org, vbabka@kernel.org, harry@kernel.org, jannh@google.com, sj@kernel.org, ziy@nvidia.com, linux-mm@kvack.org, Lorenzo Stoakes , stable@vger.kernel.org References: <20260508013728.21285-1-richard.weiyang@gmail.com> <5e9ee072-b927-41e0-ba98-c9fdf11eccbc@nvidia.com> <0aab59b8-71c5-4059-8281-5dd876946528@kernel.org> <20260512143542.izpp3gu4iqxttw3f@master> From: "David Hildenbrand (Arm)" Content-Language: en-US Autocrypt: addr=david@kernel.org; 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charset=UTF-8 Content-Transfer-Encoding: 7bit X-Rspamd-Server: rspam12 X-Rspamd-Queue-Id: 0C7684000E X-Stat-Signature: 3wafa58166uejopf183jjfn4zoafte74 X-Rspam-User: X-HE-Tag: 1778612154-349243 X-HE-Meta: U2FsdGVkX19wFetVFZ6my9e99GtuBx5ZoXJWY8A+7Z7OTs047onEBcIEP7mLby1lTWf/pKSrSwm/sIqRQXHswSiCnjqKYTt0ejN3G6Zsds6NVQz+AEmiX1blX0gwtepe+ZLtE1MyIwAPvd+XXAXnq1sgbVf4URxprBfrlmNb4a6vLKRq4ZPEviiP/xvrSwLVUskjwtiSE8HzjtUM+9SBld6Xsnyq2O+FfbgCsoDAZsJgRfk2pbT58xq1ESKDNXVvB0wLZBkf3DJip2xee4rg1JTzdQQFR7aN2NVn+LreyqAHhb8fx9r46RsPhhlgh6gUBCd7bbyf7H8t1P1EoFlnn1cpSxMTCDsIz7vOHFPcIc0LLN5jrgGkXNZ2biKpl+zY3dKs7pmpe88tVH/59EfgmJd8qCgWxIsjLwyawkEEW/+OBrKTFDCIuh48PkOGlfsxE1OsbiNRlJwVniOuk0XOUA6dJSqUhyHlIYXVffSje6ivgLFbqEgmfw+jTggQoqkekHY1mfku9w3XZUlBE+mvaEuRkLfTGJoE1UMyGaWXzGLwZ1ecIVOcihn5PQ1RwZym0TwwW2osy+z62DzJqqbT8eqf1HwGUGmCPf5kQ7BfKQmWVvjjaHHozysuVC03aeWPYigQPThBEmvNIiozhZ1cVNTHI9idmAs+YNc7kQBlNxzPS8BB3qxdSE8mRVtyEhI7P6AC8B98aJ8yDgTVAm36Jb4kt3VK2VCoKUpoXuAMgBvxkRzFsK2wV6yr+wuCYN8UbQ1lqr07Nb9eUx1V6M12VVEjDCBRNaPV251SZP4v8wCqttCQPP2Q/KQONm9u8B0HOrg4eJANezKcrGwn9vCD4n31mW1xCDbnrMR2Xq5BHj0O38VaBk5GLpYug8HM5evxiim2MJEdAa/pbtg3tEZJO14ZEcBMxxUkP2xvArc9N5q072yjSPC0aR6+TiTYYNA81dIQwj8AzIdoF2R/+o+ wTAudYmc HAg4sHI07pS0ietex/7Y0l+/NWwJPpm5h/9BhxwVgn3BS2T++d9yuUM0s1n57nuzBz/NnVYcS3e4BqSp/tbUvvwAgxLMGPbeNoWzqIJQgF8eMOf9Ssxz6JCnU41TmYjp0Gdd6sPEXkp+1XbntL81LFqggT+Eg8UB6YGEOb7v+pKdYw+/3ETytsNBTGUolq+WQAIfCTDEuEDBf0HnHEG8ECzjiWOz7GmT+0Rpx5SkqItfDxESnH8xkAMMnU6XVz3f9bAKfZTGGWCashh0wnalHiZd6G5+8XwH8yYGacwoee2ijfr02QY2stWb/Xmk8JHK7gcuVsyJB391f+l0MaLLm9xntqLdSe//+V9oiSNXvRK2T8zDQUKOCiInctw== Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: List-Subscribe: List-Unsubscribe: On 5/12/26 16:35, Wei Yang wrote: > On Tue, May 12, 2026 at 02:43:54PM +0200, David Hildenbrand (Arm) wrote: >> On 5/9/26 00:48, Balbir Singh wrote: >>> >>> Could you elaborate a more on the improper situation? >>> >>> >>> Do we need to check softleaf_is_device_private() twice, can't we hold the pmd >>> lock and check once? >> >> I think what we try to do here is, is to only grab the lock if we verified that there is something of interest in there. >> >> I wonder if we should rewrite that whole thing to just do a pmd_same() check after grabbing the lock. >> >> Something a lot cleaner like: >> >> diff --git a/mm/page_vma_mapped.c b/mm/page_vma_mapped.c >> index a4d52fdb3056..de6a255cc847 100644 >> --- a/mm/page_vma_mapped.c >> +++ b/mm/page_vma_mapped.c >> @@ -242,40 +242,28 @@ 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)) { >> - pvmw->ptl = pmd_lock(mm, pvmw->pmd); >> - pmde = *pvmw->pmd; >> - if (!pmd_present(pmde)) { >> - softleaf_t entry; >> - >> - if (!thp_migration_supported() || >> - !(pvmw->flags & PVMW_MIGRATION)) >> - return not_found(pvmw); >> - entry = softleaf_from_pmd(pmde); >> - >> - if (!softleaf_is_migration(entry) || >> - !check_pmd(softleaf_to_pfn(entry), pvmw)) >> - return not_found(pvmw); >> - return true; >> - } >> - if (likely(pmd_trans_huge(pmde))) { >> - if (pvmw->flags & PVMW_MIGRATION) >> - return not_found(pvmw); >> - if (!check_pmd(pmd_pfn(pmde), pvmw)) >> - return not_found(pvmw); >> - return true; >> - } >> - /* THP pmd was split under us: handle on pte level */ >> - spin_unlock(pvmw->ptl); >> - pvmw->ptl = NULL; >> - } else if (!pmd_present(pmde)) { >> - const softleaf_t entry = softleaf_from_pmd(pmde); >> - >> - if (softleaf_is_device_private(entry)) { >> - pvmw->ptl = pmd_lock(mm, pvmw->pmd); >> - return true; >> - } >> + if (pmd_present(pmde)) { >> + if (!pmd_leaf(pmde)) >> + goto pte_table; >> + if (pvmw->flags & PVMW_MIGRATION) >> + return not_found(pvmw); >> + if (!check_pmd(pmd_pfn(pmde), pvmw)) >> + return not_found(pvmw); >> + } else if (pmd_is_migration_entry(pmde)) { >> + softleaf_t entry = softleaf_from_pmd(pmde); >> + >> + if (!(pvmw->flags & PVMW_MIGRATION)) >> + return not_found(pvmw); >> + if (!check_pmd(softleaf_to_pfn(entry), pvmw)) >> + return not_found(pvmw); >> + } else if (pmd_is_device_private_entry(pmde)) { >> + softleaf_t entry = softleaf_from_pmd(pmde); >> >> + if (pvmw->flags & PVMW_MIGRATION) >> + return not_found(pvmw); >> + if (!check_pmd(softleaf_to_pfn(entry), pvmw)) >> + return not_found(pvmw); >> + } else { >> if ((pvmw->flags & PVMW_SYNC) && >> thp_vma_suitable_order(vma, pvmw->address, >> PMD_ORDER) && >> @@ -285,6 +273,15 @@ bool page_vma_mapped_walk(struct page_vma_mapped_walk *pvmw) >> step_forward(pvmw, PMD_SIZE); >> continue; >> } >> + >> + /* Double-check under PTL that the PMD didn't change. */ >> + pvmw->ptl = pmd_lock(mm, pvmw->pmd); >> + if (pmd_same(pmde, pmdp_get(pvmw->pmd))) >> + return true; >> + spin_unlock(pvmw->ptl); >> + pvmw->ptl = NULL; >> + goto restart; >> +pte_table: >> if (!map_pte(pvmw, &pmde, &ptl)) { >> if (!pvmw->pte) >> >> >> >> >> There is likely room to clean this up / compress it further. > > I tried to compress above logic like this, hope it could look cleaner. > > if (pmd_trans_huge(pmde) || pmd_is_valid_softleaf(pmde)) { > unsigned long pfn; > bool is_migration = pmd_is_migration_entry(pmde); > bool for_migration = !!(pvmw->flags & PVMW_MIGRATION); > > if (is_migration != for_migration) > return not_found(pvmw); > > if (pmd_trans_huge(pmde)) > pfn = pmd_pfn(pmde); > else > pfn = softleaf_to_pfn(softleaf_from_pmd(pmde)); > > if (!check_pmd(pfn, pvmw)) > return not_found(pvmw); > } else if (!pmd_present(pmde)) { It's more compact, but not necessarily cleaner. In particular, I detest pmd_trans_huge(), we should phase it out. if (pmd_present(pmde) && !pmd_leaf(pmde)) { goto pte_table; } else if (pmd_present(pmde) || pmd_is_valid_softleaf(pmde)) ... Might work as well. But once we add support for other softleaf types, we'll have to touch it again. So I'd rather just list what we actually expect. -- Cheers, David