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Howlett" , Nico Pache , Ryan Roberts , Dev Jain , Barry Song , Lance Yang , linux-fsdevel@vger.kernel.org, linux-kernel@vger.kernel.org, linux-mm@kvack.org, david@redhat.com, jane.chu@oracle.com, kernel@pankajraghav.com, syzbot+e6367ea2fdab6ed46056@syzkaller.appspotmail.com, syzkaller-bugs@googlegroups.com Subject: Re: [PATCH 2/2] mm/memory-failure: improve large block size folio handling. 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>>> - if (try_to_split_thp_page(p, false) < 0) { >>> + /* >>> + * If the folio cannot be split to order-0, kill the process, >>> + * but split the folio anyway to minimize the amount of unusable >>> + * pages. >>> + */ >>> + if (try_to_split_thp_page(p, new_order, false) || new_order) { >>> + /* get folio again in case the original one is split */ >>> + folio = page_folio(p); >> >> If original folio A is split and the after-split new folio is B (A != B), will the >> refcnt of folio A held above be missing? I.e. get_hwpoison_page() held the extra refcnt >> of folio A, but we put the refcnt of folio B below. Is this a problem or am I miss >> something? > > That's how split works. > > Zi Yan, the kernel-doc for folio_split() could use some attention. > First, it's not kernel-doc; the comment opens with /* instead of /**. Got it. > Second, it says: > > * After split, folio is left locked for caller. > > which isn't actually true, right? The folio which contains No, folio is indeed left locked. Currently folio_split() is used by truncate_inode_partial_folio() via try_folio_split() and the folio passed into truncate_inode_partial_folio() is already locked by the caller and is unlocked by the caller as well. The caller does not know anything about @split_at, thus cannot unlock the folio containing @split_at. > @split_at will be locked. Also, it will contain the additional > reference which was taken on @folio by the caller. The same for the folio reference. That is the reason we have @split_at and @lock_at for __folio_split(). I can see it is counter-intuitive. To change it, I might need your help on how to change truncate_inode_partial_folio() callers, since all of them are use @folio afterwards, without a reference, I am not sure if their uses are safe anymore. -- Best Regards, Yan, Zi