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From: Usama Arif In-Reply-To: <80683d6d-ea38-4326-af5e-e4c173bb1930@redhat.com> Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 8bit X-Migadu-Flow: FLOW_OUT X-Stat-Signature: 4kiscw8o5oi4y7wccdrhothq3t47e1kb X-Rspam-User: X-Rspamd-Queue-Id: 5D67DC000F X-Rspamd-Server: rspam12 X-HE-Tag: 1772728609-896403 X-HE-Meta: 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 jpWCIOTr D52niXYyudzhACipAvigLwtrHZNELEptVmYdHCwwX01SFeYQmpSwT3fy2FqIYwSTTpH2WcWiKaOHJ/cZNLV/i7STC8c6JQup3egLDVej2ftMFLovC9h5zAnLmRN75ygBpzEiS74Ha5uf9vZHl1TyFrAosT0dwSb6viMbTQv1Kmr1ldW5IDO4qal1FAgy34zG3HgQqSWlOWmWoLizqZSpPcOzH3HnZif6Dg28fNm+ERVHhVghpYNKGqdR3S7Aoxcezq6QEwuI4E/gYkJxh0GVdsyGlwbUW7g0QACQsN+8FNFMyW5lxWTSDtovY0Mu91N8FzBUPJ+rQoPzHIrViBWS1PNnmdEiNg7N+Bs+UVPog6Ixs3w08q856jkbUxlgGp1LgBwmlQVMSBVULwTwoBOAy81NyIGLIeewh5Try0o+k7HlFu+IGuUSnXiPJgKogqBr4ngFiKL5Zk0gpR2trJxPqKyQan86OF349194xIPnCQfKp1a85UlRgQ1O8HmFZHFFfqi1YBbteUtu57CfiAyfaZVQmuw== Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: List-Subscribe: List-Unsubscribe: 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! [1] https://gist.github.com/uarif1/65e1e816af7aa0ae38dd6ec64d62a993 [2] https://gist.github.com/uarif1/79ea9500667daa4e2ef09cb5d308f041 [3] https://gist.github.com/uarif1/8a35a6c65ba8b3a1c1dfe72dc30e821d