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folio_split_unmapped failure To: Usama Arif , Zi Yan Cc: =?UTF-8?Q?Mika_Penttil=C3=A4?= , Kiryl Shutsemau , matthew.brost@intel.com, npache@redhat.com, david@kernel.org, Usama Arif , Andrew Morton , linux-mm@kvack.org, joshua.hahnjy@gmail.com, hannes@cmpxchg.org, rakie.kim@sk.com, byungchul@sk.com, gourry@gourry.net, ying.huang@linux.alibaba.com, apopple@nvidia.com, riel@surriel.com, shakeel.butt@linux.dev, linux-kernel@vger.kernel.org, kernel-team@meta.com References: <20260304120132.3973445-1-usamaarif642@gmail.com> <5e59c077-9f06-4e45-86e1-ca696e6105b4@nvidia.com> <622eb392-8c04-473d-b42a-ecdc489799c4@linux.dev> <942f2df4-6fb5-415f-b7d4-87a83315890b@redhat.com> <80683d6d-ea38-4326-af5e-e4c173bb1930@redhat.com> <332c9e16-46c3-4e1c-898e-2cb0a87ba1fc@linux.dev> <7996d5c5-24db-4ef2-b88a-1b9d33f9e976@linux.dev> Content-Language: en-US From: Balbir Singh In-Reply-To: <7996d5c5-24db-4ef2-b88a-1b9d33f9e976@linux.dev> Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 8bit 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 WLIi3tbR 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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 3/6/26 04:00, Usama Arif wrote: > > > On 05/03/2026 16:39, Zi Yan wrote: >> On 5 Mar 2026, at 11:36, Usama Arif wrote: >> >>> On 05/03/2026 12:09, Mika Penttilä wrote: >>>> On 3/5/26 13:44, Usama Arif wrote: >>>> >>>>> >>>>> On 05/03/2026 06:09, Mika Penttilä wrote: >>>>>> Hi! >>>>>> >>>>>> On 3/5/26 01:28, Usama Arif wrote: >>>>>> >>>>>>> On 04/03/2026 22:09, Balbir Singh wrote: >>>>>>>> On 3/5/26 08:54, Zi Yan wrote: >>>>>>>>> On 4 Mar 2026, at 16:48, Balbir Singh wrote: >>>>>>>>> >>>>>>>>>> On 3/5/26 02:17, Zi Yan wrote: >>>>>>>>>>> On 4 Mar 2026, at 7:01, Usama Arif wrote: >>>>>>>>>>> >>>>>>>>>>>> From: Usama Arif >>>>>>>>>>>> >>>>>>>>>>>> migrate_vma_split_unmapped_folio() takes an extra reference via >>>>>>>>>>>> folio_get() before calling folio_split_unmapped(). On success, the >>>>>>>>>>>> split consumes this reference: __folio_freeze_and_split_unmapped() >>>>>>>>>>>> expects the +1 in its folio_ref_freeze() check, and distributes it >>>>>>>>>>>> across the resulting sub-folios via folio_ref_unfreeze(...+1), which >>>>>>>>>>>> are later balanced by folio_put() calls in __migrate_device_finalize(). >>>>>>>>>>>> >>>>>>>>>>>> If folio_split_unmapped() fails (e.g., unexpected pinning returns >>>>>>>>>>>> -EAGAIN), the function returns without calling folio_put(). The extra >>>>>>>>>>>> reference is never released. >>>>>>>>>>>> >>>>>>>>>>>> Add the missing folio_put() on the error path. >>>>>>>>>>>> >>>>>>>>>>>> Fixes: 4265d67e405a4 ("mm/migrate_device: add THP splitting during migration") >>>>>>>>>>>> Closes: https://lore.kernel.org/all/CAA1CXcDyqPPwf_-W7B+PFQtL8HdoJGCEqVsVxq7DhOUB=L4PQA@mail.gmail.com/ >>>>>>>>>>>> Reported-by: Nico Pache >>>>>>>>>>>> Signed-off-by: Usama Arif >>>>>>>>>>>> --- >>>>>>>>>>>> mm/migrate_device.c | 4 +++- >>>>>>>>>>>> 1 file changed, 3 insertions(+), 1 deletion(-) >>>>>>>>>>>> >>>>>>>>>>>> diff --git a/mm/migrate_device.c b/mm/migrate_device.c >>>>>>>>>>>> index 0a8b31939640f..351ecd9065d13 100644 >>>>>>>>>>>> --- a/mm/migrate_device.c >>>>>>>>>>>> +++ b/mm/migrate_device.c >>>>>>>>>>>> @@ -917,8 +917,10 @@ static int migrate_vma_split_unmapped_folio(struct migrate_vma *migrate, >>>>>>>>>>>> folio_get(folio); >>>>>>>>>>>> split_huge_pmd_address(migrate->vma, addr, true); >>>>>>>>>>>> ret = folio_split_unmapped(folio, 0); >>>>>>>>>>>> - if (ret) >>>>>>>>>>>> + if (ret) { >>>>>>>>>>>> + folio_put(folio); >>>>>>>>>>>> return ret; >>>>>>>>>>>> + } >>>>>>>>>>>> migrate->src[idx] &= ~MIGRATE_PFN_COMPOUND; >>>>>>>>>>>> flags = migrate->src[idx] & ((1UL << MIGRATE_PFN_SHIFT) - 1); >>>>>>>>>>>> pfn = migrate->src[idx] >> MIGRATE_PFN_SHIFT; >>>>>>>>>>>> -- >>>>>>>>>>>> 2.47.3 >>>>>>>>>>> Add Balbir, who wrote the code, to comment on this. >>>>>>>>>>> >>>>>>>>>> Thanks Zi! >>>>>>>>>> >>>>>>>>>> Just wondering if there is a reproducer for the issue and how the fix was tested? >>>>>>>>>> I expect migrate_vma_finalize() to be called for folios, even when split failed and >>>>>>>>>> drop the lock. >>>>>>>>> Does migrate_vma_finalize() do folio_put() for failed-to-split folios? >>>>>>>>> If so, how does it distinguish between split folios and failed-to-split folios? >>>>>>>>> By comparing source and destination folio orders? >>>>>>>>> >>>>>>>> We reset the MIGRATE_PFN_MIGRATE flag for failing to migrate pfns. We do a folio_put >>>>>>>> on the src in finalize, if it is split then on all the split folios as well. >>>>>>>> >>>>>>>>> What we see from migrate_vma_split_unmapped_folio() is that >>>>>>>>> it adds a refcount for all input folios, but only drops a refcount >>>>>>>>> for the split folio. Isn’t it cause failed-to-split folios to have >>>>>>>>> additional refcount? >>>>>>>>> >>>>>>> Hello! >>>>>>> >>>>>>> Thanks for reviewing everyone. So its very difficult to create a reproducer I think >>>>>>> the extra reference would need to appear after migrate_device_unmap() but before >>>>>>> folio_split_unmapped() in migrate_vma_pages()? That's hard to trigger reliably from >>>>>>> userspace. >>>>>>> >>>>>>> The fix came about when Nico indicated there might be an issue if split_huge_pmd_address >>>>>>> fails in my patch [1]. >>>>>>> >>>>>>> Below is my understanding of how refcounting is working over here step by step. I >>>>>>> might very well be wrong on this, and the refcounting is a bit all over the place >>>>>>> and I might miss a reference change somewhere so would really appreciate if someone >>>>>>> can confirm this! >>>>>>> >>>>>>> >>>>>>> 1. migrate_vma_collect_huge_pmd(): >>>>>>> a) folio_get(folio) -> +1 (collect reference) >>>>>>> 2. migrate_device_unmap(): >>>>>>> a) folio_isolate_lru() -> +1 (isolation reference) >>>>>>> b) folio_put() -> -1 (drops the collect reference) >>>>>>> >>>>>>> >>>>>>> Without this patch fix: >>>>>>> >>>>>>> 3. migrate_vma_split_unmapped_folio(): >>>>>>> a) folio_get(folio) -> +1 (split reference) >>>>>>> b) folio_split_unmapped() -> fails >>>>>>> c) Returns error — without folio_put() which is the fix >>>>>>> 4. Caller in migrate_vma_pages(): clears MIGRATE_PFN_MIGRATE | MIGRATE_PFN_COMPOUND >>>>>>> 5. __migrate_device_finalize(): sees !(src_pfns[i] & MIGRATE_PFN_MIGRATE), restores the folio: >>>>>>> a) remove_migration_ptes(src, src) — re-establishes user PTEs >>>>>>> b) folio_unlock(src) >>>>>>> c) folio_put(src) -> -1 (drops the isolation reference) >>>>>>> >>>>>>> The split reference in 3.a is never released and the folio has a permanently elevated refcount. >>>>>>> Unless I missed a folio_put somewhere for the refcount increase in folio_isolate_lru() (2.b)? >>>>>>> >>>>>>> Please let me know if this makes sense! >>>>>>> >>>>>>> [1] https://lore.kernel.org/all/CAA1CXcDyqPPwf_-W7B+PFQtL8HdoJGCEqVsVxq7DhOUB=L4PQA@mail.gmail.com/ >>>>>>> >>>>>>>> Thanks! Yes, the patch makes sense >>>>>>>> >>>>>>>> Acked-by: Balbir Singh >>>>>>>> >>>>>>>> Balbir >>>>>> I remember stumbling on this while ago also. The folio_get() in migrate_vma_split_unmapped_folio() >>>>>> is balanced with put_page() in __split_huge_pmd_locked() (freeze = true), can't fail for device pages. >>>>>> Folios at this point are unmapped but have 1 refcount from "collecting". >>>>>> After folio_split_unmapped() the refcount(s) is still 1. >>>>>> >>>>>> So it seems the code is good as is? A comment though would be good for the extra folio_get.. >>>>>> >>>>> hmm I dont think the put_page() in __split_huge_pmd_locked() is there to balance the folio_get() in >>>>> migrate_vma_split_unmapped_folio(). There are other points where split_huge_pmd_locked() is called >>>>> with freeze = true [1] and they don't get a reference before calling split_huge_pmd. >>>>> >>>>> I think the folio_put() in __split_huge_pmd_locked() freeze = true case is there as migration >>>>> entries are being installed? >>>>> >>>>> [1] https://elixir.bootlin.com/linux/v6.19.3/source/mm/rmap.c#L2334 >>>>> >>>>> >>>> Yes normally you want to drop the reference when installing migration entries but in this context >>>> you have already done the collecting for the THP folio and you want to balance with the folio_get() >>>> the put_page() to keep the refs unchanged. Is that right Balbir? >>>> >>>> --Mika >>>> >>> >>> Hi Mika, >>> >>> You are right, This patch is wrong. I tried the below diff to force folio_split_unmapped to return >>> -EAGAIN. I ran tools/testing/selftests/mm/hmm-tests -r hmm.hmm_device_private.migrate_anon_huge_err >>> to trigger the path for folio_split_unmapped. >>> >>> diff --git a/mm/huge_memory.c b/mm/huge_memory.c >>> index 8e2746ea74adf..6df33b4990a13 100644 >>> --- a/mm/huge_memory.c >>> +++ b/mm/huge_memory.c >>> @@ -4140,6 +4140,8 @@ int folio_split_unmapped(struct folio *folio, unsigned int new_order) >>> if (folio_expected_ref_count(folio) != folio_ref_count(folio) - 1) >>> return -EAGAIN; >>> >>> + return -EAGAIN; >>> + >>> local_irq_disable(); >>> ret = __folio_freeze_and_split_unmapped(folio, new_order, &folio->page, NULL, >>> NULL, false, NULL, SPLIT_TYPE_UNIFORM, >>> >>> >>> >>> I inserted a lot of traces to keep track of refcounts [1]. Without this patch, I get >>> .... >>> hmm-tests-129 [000] ..... 1.476233: __migrate_device_pages: SPLIT_UNMAPPED: folio=ffc536e2c4100000 refcount=0 AFTER error NO folio_put >>> hmm-tests-129 [000] ..... 1.476234: __migrate_device_pages: PAGES: split FAILED folio=ffc536e2c4100000 refcount=0 >>> hmm-tests-129 [000] ..... 1.476236: __migrate_device_finalize: FINALIZE[0]: src=ffc536e2c4100000 dst=ffc536e2c4100000 src==dst=1 refcount_src=1 mapcount_src=0 order_src=0 migrate=0 BEFORE remove_migration_ptes >>> hmm-tests-129 [000] ..... 1.476237: __migrate_device_finalize: FINALIZE[0]: src=ffc536e2c4100000 refcount=1 mapcount=0 AFTER remove_migration_ptes >>> hmm-tests-129 [000] ..... 1.476237: __migrate_device_finalize: FINALIZE[0]: src=ffc536e2c4100000 refcount=0 AFTER folio_put(src) >>> >>> i.e. refcount = 512, which is correct as split_huge_pmd_address was successful. Full output is >>> at [2]. >>> >>> With this patch, I get: >>> >>> BUG: Bad rss-counter state mm:00000000cfe88d5e type:MM_FILEPAGES val:-511 Comm:bash Pid:63 >>> BUG: Bad rss-counter state mm:00000000cfe88d5e type:MM_ANONPAGES val:511 Comm:bash Pid:63 >>> ... >>> hmm-tests-129 [000] ..... 1.468315: __migrate_device_pages: SPLIT_UNMAPPED: folio=ffed210c840f0000 refcount=1 AFTER error folio_put FIX PRESENT >>> hmm-tests-129 [000] ..... 1.468315: __migrate_device_pages: PAGES: split FAILED folio=ffed210c840f0000 refcount=1 >>> hmm-tests-129 [000] ..... 1.468318: __migrate_device_finalize: FINALIZE[0]: src=ffed210c840f0000 dst=ffed210c840f0000 src==dst=1 refcount_src=1 mapcount_src=0 order_src=9 migrate=0 BEFORE remove_migration_ptes >>> hmm-tests-129 [000] ..... 1.468357: __migrate_device_finalize: FINALIZE[0]: src=ffed210c840f0000 refcount=513 mapcount=512 AFTER remove_migration_ptes >>> hmm-tests-129 [000] ..... 1.468357: __migrate_device_finalize: FINALIZE[0]: src=ffed210c840f0000 refcount=512 AFTER folio_put(src) >>> >>> refcount=0 means the folio would be freed which is not correct. The full output is at [3]. >>> >>> Thank you for clearing this up! >> >> Thank you for doing the investigation. Can you send a patch to add a comment >> in migrate_vma_split_unmapped_folio() about this to avoid the confusion >> in the future? >> > > Yeah this was really confusing. > > Does something like below look good? > > diff --git a/mm/migrate_device.c b/mm/migrate_device.c > index 78c7acf024615..a302f9d3ce921 100644 > --- a/mm/migrate_device.c > +++ b/mm/migrate_device.c > @@ -910,6 +910,11 @@ static int migrate_vma_split_unmapped_folio(struct migrate_vma *migrate, > > folio_get(folio); > split_huge_pmd_address(migrate->vma, addr, true); > + /* > + * split_huge_pmd_address consumes the folio_get reference above. > + * Therefore no folio_put is needed on the folio_split_unmapped > + * error path. > + */ > ret = folio_split_unmapped(folio, 0); > if (ret) > return ret; > The comment should come above folio_get and state that /* * folio_get() is required because this reference is to be consumed * by split_huge_pmd_address() for frozen entries */ >>> >>> >>> [1] https://gist.github.com/uarif1/65e1e816af7aa0ae38dd6ec64d62a993 >>> [2] https://gist.github.com/uarif1/79ea9500667daa4e2ef09cb5d308f041 >>> [3] https://gist.github.com/uarif1/8a35a6c65ba8b3a1c1dfe72dc30e821d >> >> >> Best Regards, >> Yan, Zi > > @Usama have you figured out where the original issue you are seeing stems from? Balbir