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Fri, 26 Jun 2026 19:07:20 -0700 (PDT) Date: Sat, 27 Jun 2026 02:07:19 +0000 From: Wei Yang To: "David Hildenbrand (Arm)" Cc: Wei Yang , akpm@linux-foundation.org, ljs@kernel.org, riel@surriel.com, liam@infradead.org, vbabka@kernel.org, harry@kernel.org, jannh@google.com, ziy@nvidia.com, sj@kernel.org, balbirs@nvidia.com, linux-mm@kvack.org, linux-kernel@vger.kernel.org, stable@vger.kernel.org, Lance Yang Subject: Re: [Patch mm-hotfixes v4] mm/page_vma_mapped: fix device-private PMD handling Message-ID: <20260627020719.ipzfrlhfbvr6ac35@master> Reply-To: Wei Yang References: <20260624065353.1622-1-richard.weiyang@gmail.com> 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=us-ascii Content-Disposition: inline In-Reply-To: User-Agent: NeoMutt/20170113 (1.7.2) On Fri, Jun 26, 2026 at 12:07:56PM +0200, David Hildenbrand (Arm) wrote: >On 6/24/26 08:53, Wei Yang wrote: [...] > >This is extremely hard to review given the existing crap handling here. I'm >really sorry, but it makes my head hurt (I'm not kidding :) ). > >It's completely unclear why we only have to check for a subset of the cases >after taking the lock. > >Could we simply extend the existing migration pmd handling and leave the >!pmd_present() case for pmd_none()? > >That leaves no question to "which transitions are actually allowed", including >"could we accidentally assume something is a page table when really it isn't". > > >So what about something like the following? > >The "thp_migration_supported()" is not required when checking for >pmd_is_migration_entry(), as that defaults to "false" when not compiled in. > >Untested: > Hi David I did a little adjustment like below. Want to check with you at first. > >>>From 048ecd33673ec649e168fbbb97749a7c0e344fcd Mon Sep 17 00:00:00 2001 >From: "David Hildenbrand (Arm)" >Date: Fri, 26 Jun 2026 12:03:40 +0200 >Subject: [PATCH] tmp > >Signed-off-by: David Hildenbrand (Arm) >--- > mm/page_vma_mapped.c | 29 +++++++++++++++++------------ > 1 file changed, 17 insertions(+), 12 deletions(-) > >diff --git a/mm/page_vma_mapped.c b/mm/page_vma_mapped.c >index 2ccbabfb2cc17..ed2a23a90e8dd 100644 >--- a/mm/page_vma_mapped.c >+++ b/mm/page_vma_mapped.c >@@ -243,21 +243,31 @@ 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; > How about: const softleaf_t entry = softleaf_from_pmd(pmde); >- if (!thp_migration_supported() || >- !(pvmw->flags & PVMW_MIGRATION)) >+ if (!(pvmw->flags & PVMW_MIGRATION)) > return not_found(pvmw); > entry = softleaf_from_pmd(pmde); could be removed. >+ 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; The same. > >- if (!softleaf_is_migration(entry) || >- !check_pmd(softleaf_to_pfn(entry), pvmw)) >+ 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_present(pmde) ){ >+ return not_found(pvmw); > } > if (likely(pmd_trans_huge(pmde))) { > if (pvmw->flags & PVMW_MIGRATION) How about merge this with above? And put at the first case? Below is what it looks like: 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 (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; } else if (pmd_is_migration_entry(pmde)) { const 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); return true; } else if (pmd_is_device_private_entry(pmde)) { const 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); return true; } else if (!pmd_present(pmde)) { return not_found(pvmw); } /* THP pmd was split under us: handle on pte level */ spin_unlock(pvmw->ptl); pvmw->ptl = NULL; } else if (!pmd_present(pmde)) { Test with split_huge_page_test/khugepaged/hmm-test/migration in selftets, looks good. -- Wei Yang Help you, Help me