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[2003:cb:c707:7600:5c18:5a7d:c5b7:e7a9]) by smtp.gmail.com with ESMTPSA id ck12-20020a5d5e8c000000b0033d9f0dcb35sm11920501wrb.87.2024.02.27.07.01.01 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Tue, 27 Feb 2024 07:01:02 -0800 (PST) Message-ID: <42be658c-cb13-4001-aae4-8d8275a84038@redhat.com> Date: Tue, 27 Feb 2024 16:01:01 +0100 MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH] mm/huge_memory: fix swap entry values of tail pages of THP To: Zi Yan Cc: Matthew Wilcox , Charan Teja Kalla , gregkh@linuxfoundation.org, akpm@linux-foundation.org, vbabka@suse.cz, dhowells@redhat.com, surenb@google.com, linux-mm@kvack.org, linux-kernel@vger.kernel.org, # see patch description , Huang Ying , Naoya Horiguchi References: <1707814102-22682-1-git-send-email-quic_charante@quicinc.com> <8620c1a0-e091-46e9-418a-db66e621b9c4@quicinc.com> <845ca78f-913b-4a92-8b40-ff772a7ad333@redhat.com> <30ea073d-0ccf-46e1-954d-e22f5cbf69f7@redhat.com> <1ABD022A-35FC-4A6E-ADAD-36F3D745FB91@nvidia.com> From: David Hildenbrand Autocrypt: addr=david@redhat.com; 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charset=UTF-8; format=flowed Content-Transfer-Encoding: 8bit X-Rspamd-Server: rspam08 X-Rspamd-Queue-Id: 3C84340082 X-Stat-Signature: obqduo7dw7fej67g8zcfdy43w669qy7q X-Rspam-User: X-HE-Tag: 1709046069-447597 X-HE-Meta: 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 pyBpTs/N cXo5VwO/HR3mCD8jlOJRBnV9vLWgAxsj6wMyPfL5IGJXNLTlM7gElyLBpuIhDQHDz/MXhHoFg+cHp7xj5EJSJMxwAS/tPo31mF9y3NTpVXScH4ZfcUM1KylYutrW9AmXUng+sU3Sw5HjlE9k+LegmhqMhexgFlu5nTBJXESh+p4FTAkgBqM5t4ZeaeXo3x1cfUsGacw5bqAU5ANaOwXHMWQ4Wk3VmrIBVuUKd12ux/1lmFVS7u5T2MKTnNTSX9KbRP1j3ddPhEiVk/G1cT5b7tE74TqS80faXmgBGiC3ztmKuSdJx9wp8quIjGJmWr/0+GNRk0yTkYfERbbeIRsvEeSPPmSfDpG8SzMpOS3xl3zEVR+JCpzukKeGtTGZz+SCIvEcYZua2xjsFniXGgoW8WCFrsWwTXSnh1pABJsMdQ80FXIoovvNwCSBi+Un4Kf2Ki2l/RHWscufEYvJeP3C+P5UrLA== X-Bogosity: Ham, tests=bogofilter, spamicity=0.000000, version=1.2.4 Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: List-Subscribe: List-Unsubscribe: On 27.02.24 15:52, Zi Yan wrote: > On 27 Feb 2024, at 9:11, David Hildenbrand wrote: > >> On 14.02.24 15:34, David Hildenbrand wrote: >>> On 14.02.24 15:18, Matthew Wilcox wrote: >>>> On Wed, Feb 14, 2024 at 12:04:10PM +0530, Charan Teja Kalla wrote: >>>>>> 1) Is it broken in 5.15? Did you actually try to reproduce or is this >>>>>>    just a guess? >>>>>> >>>>> >>>>> We didn't run the tests with THP enabled on 5.15, __so we didn't >>>>> encounter this issue__ on older to 6.1 kernels. >>>>> >>>>> I mentioned that issue exists is based on my understanding after code >>>>> walk through. To be specific, I just looked to the >>>>> migrate_pages()->..->migrate_page_move_mapping() & >>>>> __split_huge_page_tail() where the ->private field of thp sub-pages is >>>>> not filled with swap entry. If it could have set, I think these are the >>>>> only places where it would have done, per my understanding. CMIW. >>>> >>>> I think you have a misunderstanding. David's patch cfeed8ffe55b (part >>>> of 6.6) _stopped_ us using the tail ->private entries. So in 6.1, these >>>> tail pages should already have page->private set, and I don't understand >>>> what you're fixing. >>> >>> I think the issue is, that migrate_page_move_mapping() / >>> folio_migrate_mapping() would update ->private for a folio in the >>> swapcache (head page) >>> >>> newfolio->private = folio_get_private(folio); >>> >>> but not the ->private of the tail pages. >>> >>> So once you migrate a THP that is in the swapcache, ->private of the >>> tail pages would not be migrated and, therefore, be stale/wrong. >>> >>> Even before your patch that was the case. >>> >>> Looking at migrate_page_move_mapping(), we had: >>> >>> if (PageSwapBacked(page)) { >>> __SetPageSwapBacked(newpage); >>> if (PageSwapCache(page)) { >>> SetPageSwapCache(newpage); >>> set_page_private(newpage, page_private(page)); >>> } >>> } else { >>> VM_BUG_ON_PAGE(PageSwapCache(page), page); >>> } >>> >>> >>> I don't immediately see where the tail pages would similarly get updated >>> (via set_page_private). >>> >>> With my patch the problem is gone, because the tail page entries don't >>> have to be migrated, because they are unused. >>> >>> >>> Maybe this was an oversight from THP_SWAP -- 38d8b4e6bdc8 ("mm, THP, >>> swap: delay splitting THP during swap out"). >>> >>> It did update __add_to_swap_cache(): >>> >>> for (i = 0; i < nr; i++) { >>> set_page_private(page + i, entry.val + i); >>> error = radix_tree_insert(&address_space->page_tree, >>> idx + i, page + i); >>> if (unlikely(error)) >>> break; >>> } >>> >>> and similarly __delete_from_swap_cache(). >>> >>> But I don't see any updates to migration code. >>> >>> Now, it could be that THP migration was added later (post 2017), in that >>> case the introducing commit would not have been 38d8b4e6bdc8. >>> >> >> Let's continue: >> >> The introducing commit is likely either >> >> (1) 38d8b4e6bdc87 ("mm, THP, swap: delay splitting THP during swap out") >> >> That one added THP_SWAP, but THP migration wasn't supported yet AFAIKS. >> >> -> v4.13 >> >> (2) 616b8371539a6 ("mm: thp: enable thp migration in generic path") > > I think this is the one, since it makes THP entering migrate_page_move_mapping() > possible. > >> >> Or likely any of the following that actually allocate THP for migration: >> >> 8135d8926c08e mm: memory_hotplug: memory hotremove supports thp migration >> e8db67eb0ded3 mm: migrate: move_pages() supports thp migration >> c8633798497ce mm: mempolicy: mbind and migrate_pages support thp migration >> >> That actually enable THP migration. >> >> -> v4.14 >> >> >> So likely we'd have to fix the stable kernels: >> >> 4.19 >> 5.4 >> 5.10 >> 5.15 >> 6.1 >> >> That's a lot of pre-folio code. A backport of my series likely won't really make any sense. >> >> Staring at 4.19.307 code base, we likely have to perform a stable-only fix that properly handles the swapcache of compoud pages in migrate_page_move_mapping(). > > Something like (applies to v4.19.307): > > diff --git a/mm/migrate.c b/mm/migrate.c > index 171573613c39..59878459c28c 100644 > --- a/mm/migrate.c > +++ b/mm/migrate.c > @@ -514,8 +514,13 @@ int migrate_page_move_mapping(struct address_space *mapping, > if (PageSwapBacked(page)) { > __SetPageSwapBacked(newpage); > if (PageSwapCache(page)) { > + int i; > + > SetPageSwapCache(newpage); > - set_page_private(newpage, page_private(page)); > + for (i = 0; i < (1 << compound_order(page)); i++) { > + set_page_private(newpage + i, > + page_private(page + i)); > + } > } > } else { > VM_BUG_ON_PAGE(PageSwapCache(page), page); I'm wondering if there is a swapcache update missing as well. -- Cheers, David / dhildenb