linux-kernel - Re: [PATCH v2 03/12] mempolicy: fix migrate_pages(2) syscall return nr

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Message-ID: <87o7halwo4.fsf@yhuang6-desk2.ccr.corp.intel.com>
Date:   Sat, 07 Oct 2023 15:27:23 +0800
From:   "Huang, Ying" <ying.huang@...el.com>
To:     Hugh Dickins <hughd@...gle.com>
Cc:     Andrew Morton <akpm@...ux-foundation.org>,
        Andi Kleen <ak@...ux.intel.com>,
        Christoph Lameter <cl@...ux.com>,
        Matthew Wilcox <willy@...radead.org>,
        Mike Kravetz <mike.kravetz@...cle.com>,
        David Hildenbrand <david@...hat.com>,
        Suren Baghdasaryan <surenb@...gle.com>,
        Yang Shi <shy828301@...il.com>,
        Sidhartha Kumar <sidhartha.kumar@...cle.com>,
        Vishal Moola <vishal.moola@...il.com>,
        Kefeng Wang <wangkefeng.wang@...wei.com>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Tejun Heo <tj@...nel.org>,
        Mel Gorman <mgorman@...hsingularity.net>,
        Michal Hocko <mhocko@...e.com>, linux-kernel@...r.kernel.org,
        linux-mm@...ck.org
Subject: Re: [PATCH v2 03/12] mempolicy: fix migrate_pages(2) syscall return
 nr_failed

Hugh Dickins <hughd@...gle.com> writes:

> "man 2 migrate_pages" says "On success migrate_pages() returns the number
> of pages that could not be moved".  Although 5.3 and 5.4 commits fixed
> mbind(MPOL_MF_STRICT|MPOL_MF_MOVE*) to fail with EIO when not all pages
> could be moved (because some could not be isolated for migration),
> migrate_pages(2) was left still reporting only those pages failing at the
> migration stage, forgetting those failing at the earlier isolation stage.
>
> Fix that by accumulating a long nr_failed count in struct queue_pages,
> returned by queue_pages_range() when it's not returning an error, for
> adding on to the nr_failed count from migrate_pages() in mm/migrate.c.
> A count of pages?  It's more a count of folios, but changing it to pages
> would entail more work (also in mm/migrate.c): does not seem justified.
>
> queue_pages_range() itself should only return -EIO in the "strictly
> unmovable" case (STRICT without any MOVEs): in that case it's best to
> break out as soon as nr_failed gets set; but otherwise it should continue
> to isolate pages for MOVing even when nr_failed - as the mbind(2) manpage
> promises.
>
> There's a case when nr_failed should be incremented when it was missed:
> queue_folios_pte_range() and queue_folios_hugetlb() count the transient
> migration entries, like queue_folios_pmd() already did.  And there's a
> case when nr_failed should not be incremented when it would have been:
> in meeting later PTEs of the same large folio, which can only be isolated
> once: fixed by recording the current large folio in struct queue_pages.
>
> Clean up the affected functions, fixing or updating many comments.  Bool
> migrate_folio_add(), without -EIO: true if adding, or if skipping shared
> (but its arguable folio_estimated_sharers() heuristic left unchanged).
> Use MPOL_MF_WRLOCK flag to queue_pages_range(), instead of bool lock_vma.
> Use explicit STRICT|MOVE* flags where queue_pages_test_walk() checks for
> skipping, instead of hiding them behind MPOL_MF_VALID.
>
> Signed-off-by: Hugh Dickins <hughd@...gle.com>
> Reviewed-by: Matthew Wilcox (Oracle) <willy@...radead.org>

Thanks!  Feel free to add

Reviewed-by: "Huang, Ying" <ying.huang@...el.com>

--
Best Regards,
Huang, Ying