linux-kernel - Re: [PATCH 1/2] SHM_UNLOCK: fix long unpreemptible section

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Message-ID: <4F08022C.1020208@gmail.com>
Date:	Sat, 07 Jan 2012 03:28:28 -0500
From:	KOSAKI Motohiro <kosaki.motohiro@...il.com>
To:	Hugh Dickins <hughd@...gle.com>
CC:	Andrew Morton <akpm@...ux-foundation.org>,
	Minchan Kim <minchan.kim@...il.com>,
	Rik van Riel <riel@...hat.com>,
	Shaohua Li <shaohua.li@...el.com>,
	Eric Dumazet <eric.dumazet@...il.com>,
	Johannes Weiner <hannes@...xchg.org>,
	Michel Lespinasse <walken@...gle.com>,
	linux-kernel@...r.kernel.org, linux-mm@...ck.org
Subject: Re: [PATCH 1/2] SHM_UNLOCK: fix long unpreemptible section

(1/6/12 4:10 PM), Hugh Dickins wrote:
> scan_mapping_unevictable_pages() is used to make SysV SHM_LOCKed pages
> evictable again once the shared memory is unlocked.  It does this with
> pagevec_lookup()s across the whole object (which might occupy most of
> memory), and takes 300ms to unlock 7GB here.  A cond_resched() every
> PAGEVEC_SIZE pages would be good.
>
> However, KOSAKI-san points out that this is called under shmem.c's
> info->lock, and it's also under shm.c's shm_lock(), both spinlocks.
> There is no strong reason for that: we need to take these pages off
> the unevictable list soonish, but those locks are not required for it.
>
> So move the call to scan_mapping_unevictable_pages() from shmem.c's
> unlock handling up to shm.c's unlock handling.  Remove the recently
> added barrier, not needed now we have spin_unlock() before the scan.
>
> Use get_file(), with subsequent fput(), to make sure we have a
> reference to mapping throughout scan_mapping_unevictable_pages():
> that's something that was previously guaranteed by the shm_lock().
>
> Remove shmctl's lru_add_drain_all(): we don't fault in pages at
> SHM_LOCK time, and we lazily discover them to be Unevictable later,
> so it serves no purpose for SHM_LOCK; and serves no purpose for
> SHM_UNLOCK, since pages still on pagevec are not marked Unevictable.
>
> The original code avoided redundant rescans by checking VM_LOCKED
> flag at its level: now avoid them by checking shp's SHM_LOCKED.
>
> The original code called scan_mapping_unevictable_pages() on a
> locked area at shm_destroy() time: perhaps we once had accounting
> cross-checks which required that, but not now, so skip the overhead
> and just let inode eviction deal with them.
>
> Put check_move_unevictable_page() and scan_mapping_unevictable_pages()
> under CONFIG_SHMEM (with stub for the TINY case when ramfs is used),
> more as comment than to save space; comment them used for SHM_UNLOCK.
>
> Signed-off-by: Hugh Dickins<hughd@...gle.com>
> Cc: stable@...r.kernel.org [back to 2.6.32 but will need respins]

Looks completely make sense.
	Reviewed-by: KOSAKI Motohiro <kosaki.motohiro@...fujitsu.com>

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