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Tue, 12 May 2026 16:14:43 -0700 (PDT) Date: Tue, 12 May 2026 23:14:42 +0000 From: Wei Yang To: Balbir Singh Cc: "David Hildenbrand (Arm)" , Wei Yang , 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 Subject: Re: [PATCH] mm/page_vma_mapped: revalidate and do proper check before return device-private pmd Message-ID: <20260512231442.53qwj37fbykp2qus@master> Reply-To: Wei Yang 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> <113dddc5-27e3-4e9e-a90c-f076a4629f51@kernel.org> <9a56d762-ebe5-429e-9fc8-a9c9e5d0d434@nvidia.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <9a56d762-ebe5-429e-9fc8-a9c9e5d0d434@nvidia.com> User-Agent: NeoMutt/20170113 (1.7.2) X-Rspamd-Server: rspam09 X-Rspamd-Queue-Id: D66C91C0004 X-Rspam-User: X-Stat-Signature: yddth6qe8mynmd7h39s1qgrpxj53rxkp X-HE-Tag: 1778627686-841388 X-HE-Meta: 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 3G58HHKW 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 Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: List-Subscribe: List-Unsubscribe: On Wed, May 13, 2026 at 09:03:47AM +1000, Balbir Singh wrote: >On 5/13/26 04:55, David Hildenbrand (Arm) wrote: >> 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); >>> > >I got some time to look at PVMW_MIGRATION, remove_migration_ptes >is invoked for device private pages, would we want them to skip >device private pmd pages? > Not get you clearly. You mean skip device-private pmd page in remove_migration_ptes()? >>> 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. >> > >Balbir -- Wei Yang Help you, Help me