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From:   Michel Lespinasse <walken@google.com>
Date:   Fri, 14 Aug 2020 01:35:37 -0700
Message-ID: <CANN689FtCsC71cjAjs0GPspOhgo_HRj+diWsoU1wr98YPktgWg@mail.gmail.com>
Subject: Re: [PATCH v2 2/2] mm: proc: smaps_rollup: do not stall write
 attempts on mmap_lock
To:     Chinwen Chang <chinwen.chang@mediatek.com>
Cc:     Matthias Brugger <matthias.bgg@gmail.com>,
        Andrew Morton <akpm@linux-foundation.org>,
        Vlastimil Babka <vbabka@suse.cz>,
        Daniel Jordan <daniel.m.jordan@oracle.com>,
        Davidlohr Bueso <dbueso@suse.de>,
        Alexey Dobriyan <adobriyan@gmail.com>,
        "Matthew Wilcox (Oracle)" <willy@infradead.org>,
        Jason Gunthorpe <jgg@ziepe.ca>,
        Steven Price <steven.price@arm.com>,
        Song Liu <songliubraving@fb.com>,
        Jimmy Assarsson <jimmyassarsson@gmail.com>,
        Huang Ying <ying.huang@intel.com>,
        LKML <linux-kernel@vger.kernel.org>,
        linux-arm-kernel@lists.infradead.org,
        linux-mediatek@lists.infradead.org, linux-fsdevel@vger.kernel.org,
        wsd_upstream@mediatek.com
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On Wed, Aug 12, 2020 at 7:13 PM Chinwen Chang
<chinwen.chang@mediatek.com> wrote:
> smaps_rollup will try to grab mmap_lock and go through the whole vma
> list until it finishes the iterating. When encountering large processes,
> the mmap_lock will be held for a longer time, which may block other
> write requests like mmap and munmap from progressing smoothly.
>
> There are upcoming mmap_lock optimizations like range-based locks, but
> the lock applied to smaps_rollup would be the coarse type, which doesn't
> avoid the occurrence of unpleasant contention.
>
> To solve aforementioned issue, we add a check which detects whether
> anyone wants to grab mmap_lock for write attempts.

I think your retry mechanism still doesn't handle all cases. When you
get back the mmap lock, the address where you stopped last time could
now be in the middle of a vma. I think the consistent thing to do in
that case would be to retry scanning from the address you stopped at,
even if it's not on a vma boundary anymore. You may have to change
smap_gather_stats to support that, though.