linux-kernel - [PATCH v4 0/3] Try to release mmap_lock temporarily in smaps

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Date:   Tue, 18 Aug 2020 09:58:15 +0800
From:   Chinwen Chang <chinwen.chang@...iatek.com>
To:     Matthias Brugger <matthias.bgg@...il.com>,
        Michel Lespinasse <walken@...gle.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Vlastimil Babka <vbabka@...e.cz>,
        Daniel Jordan <daniel.m.jordan@...cle.com>,
        Davidlohr Bueso <dbueso@...e.de>,
        Chinwen Chang <chinwen.chang@...iatek.com>,
        Alexey Dobriyan <adobriyan@...il.com>,
        "Matthew Wilcox (Oracle)" <willy@...radead.org>,
        Jason Gunthorpe <jgg@...pe.ca>,
        Steven Price <steven.price@....com>,
        Song Liu <songliubraving@...com>,
        Jimmy Assarsson <jimmyassarsson@...il.com>,
        Huang Ying <ying.huang@...el.com>,
        Daniel Kiss <daniel.kiss@....com>,
        Laurent Dufour <ldufour@...ux.ibm.com>
CC:     <linux-kernel@...r.kernel.org>,
        <linux-arm-kernel@...ts.infradead.org>,
        <linux-mediatek@...ts.infradead.org>,
        <linux-fsdevel@...r.kernel.org>, <wsd_upstream@...iatek.com>
Subject: [PATCH v4 0/3] Try to release mmap_lock temporarily in smaps_rollup

Recently, we have observed some janky issues caused by unpleasantly long
contention on mmap_lock which is held by smaps_rollup when probing large
processes. To address the problem, we let smaps_rollup detect if anyone
wants to acquire mmap_lock for write attempts. If yes, just release the
lock temporarily to ease the contention.

smaps_rollup is a procfs interface which allows users to summarize the
process's memory usage without the overhead of seq_* calls. Android uses it
to sample the memory usage of various processes to balance its memory pool
sizes. If no one wants to take the lock for write requests, smaps_rollup
with this patch will behave like the original one.

Although there are on-going mmap_lock optimizations like range-based locks,
the lock applied to smaps_rollup would be the coarse one, which is hard to
avoid the occurrence of aforementioned issues. So the detection and
temporary release for write attempts on mmap_lock in smaps_rollup is still
necessary.

Change since v1:
- If current VMA is freed after dropping the lock, it will return
- incomplete result. To fix this issue, refine the code flow as
- suggested by Steve. [1]

Change since v2:
- When getting back the mmap lock, the address where you stopped last
- time could now be in the middle of a vma. Add one more check to handle
- this case as suggested by Michel. [2]

Change since v3:
- last_stopped is easily confused with last_vma_end. Replace it with
- a direct call to smap_gather_stats(vma, &mss, last_vma_end) as
- suggested by Steve. [3]

[1] https://lore.kernel.org/lkml/bf40676e-b14b-44cd-75ce-419c70194783@arm.com/
[2] https://lore.kernel.org/lkml/CANN689FtCsC71cjAjs0GPspOhgo_HRj+diWsoU1wr98YPktgWg@mail.gmail.com/
[3] https://lore.kernel.org/lkml/db0d40e2-72f3-09d5-c162-9c49218f128f@arm.com/


Chinwen Chang (3):
  mmap locking API: add mmap_lock_is_contended()
  mm: smaps*: extend smap_gather_stats to support specified beginning
  mm: proc: smaps_rollup: do not stall write attempts on mmap_lock

 fs/proc/task_mmu.c        | 96 +++++++++++++++++++++++++++++++++++----
 include/linux/mmap_lock.h |  5 ++
 2 files changed, 92 insertions(+), 9 deletions(-)