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Mon, 09 Feb 2026 19:23:06 -0800 (PST) Date: Tue, 10 Feb 2026 03:23:04 +0000 From: Wei Yang To: Lorenzo Stoakes Cc: Wei Yang , akpm@linux-foundation.org, david@kernel.org, riel@surriel.com, Liam.Howlett@oracle.com, vbabka@suse.cz, harry.yoo@oracle.com, jannh@google.com, gavinguo@igalia.com, baolin.wang@linux.alibaba.com, ziy@nvidia.com, linux-mm@kvack.org, Lance Yang , stable@vger.kernel.org Subject: Re: [Patch v3] mm/huge_memory: fix early failure try_to_migrate() when split huge pmd for shared thp Message-ID: <20260210032304.j4k5izweewouabqb@master> Reply-To: Wei Yang References: <20260205033113.30724-1-richard.weiyang@gmail.com> Precedence: bulk X-Mailing-List: stable@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 Mon, Feb 09, 2026 at 05:08:16PM +0000, Lorenzo Stoakes wrote: >On Thu, Feb 05, 2026 at 03:31:13AM +0000, Wei Yang wrote: >> Commit 60fbb14396d5 ("mm/huge_memory: adjust try_to_migrate_one() and >> split_huge_pmd_locked()") return false unconditionally after >> split_huge_pmd_locked() which may fail early during try_to_migrate() for >> shared thp. This will lead to unexpected folio split failure. > >I think this could be put more clearly. 'When splitting a PMD THP migration >entry in try_to_migrate_one() in a rmap walk invoked by try_to_migrate() when split_huge_pmd_locked() could split a PMD THP migration entry, but here we expect a PMD THP normal entry. >TTU_SPLIT_HUGE_PMD is specified.' or something like that. > >> >> One way to reproduce: >> >> Create an anonymous thp range and fork 512 children, so we have a >> thp shared mapped in 513 processes. Then trigger folio split with >> /sys/kernel/debug/split_huge_pages debugfs to split the thp folio to >> order 0. > >I think you should explain the issue before the repro. This is just confusing >things. Mention the repro _afterwards_. > OK, will move afterwards. >> >> Without the above commit, we can successfully split to order 0. >> With the above commit, the folio is still a large folio. >> >> The reason is the above commit return false after split pmd > >This sentence doesn't really make sense. Returns false where? And under what >circumstances? > >I'm having to look through 60fbb14396d5 to understand this which isn't a good >sign. > >'This patch adjusted try_to_migrate_one() to, when a PMD-mapped THP migration I am afraid the original intention of commit 60fbb14396d5 is not just for migration entry. >entry is found, and TTU_SPLIT_HUGE_PMD is specified (for example, via >unmap_folio()), exit the walk and return false unconditionally'. > >> unconditionally in the first process and break try_to_migrate(). >> >> On memory pressure or failure, we would try to reclaim unused memory or >> limit bad memory after folio split. If failed to split it, we will leave > >Limit bad memory? What does that mean? Also should be If '_we_' or '_it_' or >something like that. > What I want to mean is in memory_failure() we use try_to_split_thp_page() and the PG_has_hwpoisoned bit is only set in the after-split folio contains @split_at. >> some more memory unusable than expected. > >'We will leave some more memory unusable than expected' is super unclear. > >You mean we will fail to migrate THP entries at the PTE level? > No. Hmm... I would like to clarify before continue. This fix is not to fix migration case. This is to fix folio split for a shared mapped PMD THP. Current folio split leverage migration entry during split anonymous folio. So the action here is not to migrate it. I am a little lost here. >Can we say this instead please? > >> >> The tricky thing in above reproduce method is current debugfs interface >> leverage function split_huge_pages_pid(), which will iterate the whole >> pmd range and do folio split on each base page address. This means it >> will try 512 times, and each time split one pmd from pmd mapped to pte >> mapped thp. If there are less than 512 shared mapped process, >> the folio is still split successfully at last. But in real world, we >> usually try it for once. > >This whole sentence could be dropped I think I don't think it adds anything. > >And you're really confusing the issue by dwelling on this I think. > >You need to restart the walk in this case in order for the PTEs to be correctly >handled right? > >Can you explain why we can't just essentially revert 60fbb14396d5? Or at least >the bit that did this change? > >Also is unmap_folio() the only caller with TTU_SPLIT_HUGE_PMD as the comment >that was deleted by 60fbb14396d5 implied? Or are there others? If it is, please >mention the commit msg. > > >> >> This patch fixes this by restart page_vma_mapped_walk() after >> split_huge_pmd_locked(). We cannot simply return "true" to fix the >> problem, as that would affect another case: > >I mean how would it fix the problem to incorrectly have it return true when the >walk had not in fact completed? > >I'm not sure why you're dwelling on this idea in the commit msg? > >> split_huge_pmd_locked()->folio_try_share_anon_rmap_pmd() can failed and >> leave the folio mapped through PTEs; we would return "true" from >> try_to_migrate_one() in that case as well. While that is mostly >> harmless, we could end up walking the rmap, wasting some cycles. > >I mean I think we can just drop this whole paragraph no? > >You might think I'm being picky about the commit msg here, but as is I find it >pretty much incomprehensible and that's not helpful if we have to go back and >read this in future. > Never mind. A clearer and comprehensive change log is helpful for all. And my English is not native language, so your suggestion helps a lot. >> >> Fixes: 60fbb14396d5 ("mm/huge_memory: adjust try_to_migrate_one() and split_huge_pmd_locked()") >> Signed-off-by: Wei Yang >> Reviewed-by: Baolin Wang >> Reviewed-by: Zi Yan >> Tested-by: Lance Yang >> Reviewed-by: Lance Yang >> Reviewed-by: Gavin Guo >> Acked-by: David Hildenbrand (arm) >> Cc: Gavin Guo >> Cc: "David Hildenbrand (Red Hat)" >> Cc: Zi Yan >> Cc: Baolin Wang >> Cc: Lance Yang >> Cc: >> >> --- >> v3: >> * gather RB >> * adjust the commit log and comment per David > >Clearly not enough :) > >> * add userspace-visible runtime effect in change log > >Which one was that? > >> v2: >> * restart page_vma_mapped_walk() after split_huge_pmd_locked() >> --- >> mm/rmap.c | 12 +++++++++--- >> 1 file changed, 9 insertions(+), 3 deletions(-) >> >> diff --git a/mm/rmap.c b/mm/rmap.c >> index 618df3385c8b..1041a64b8e6b 100644 >> --- a/mm/rmap.c >> +++ b/mm/rmap.c >> @@ -2446,11 +2446,17 @@ static bool try_to_migrate_one(struct folio *folio, struct vm_area_struct *vma, >> __maybe_unused pmd_t pmdval; >> >> if (flags & TTU_SPLIT_HUGE_PMD) { >> + /* >> + * split_huge_pmd_locked() might leave the >> + * folio mapped through PTEs. Retry the walk >> + * so we can detect this scenario and properly >> + * abort the walk. >> + */ > >This comment is a lot clearer than the commit msg :) > >> split_huge_pmd_locked(vma, pvmw.address, >> pvmw.pmd, true); >> - ret = false; >> - page_vma_mapped_walk_done(&pvmw); >> - break; >> + flags &= ~TTU_SPLIT_HUGE_PMD; >> + page_vma_mapped_walk_restart(&pvmw); >> + continue; > >This logic does lok reasonable. > >> } >> #ifdef CONFIG_ARCH_ENABLE_THP_MIGRATION >> pmdval = pmdp_get(pvmw.pmd); >> -- >> 2.34.1 >> > >Cheers, Lorenzo -- Wei Yang Help you, Help me