Messages in this thread | | | Date | Sat, 07 Jan 2012 03:28:28 -0500 | From | KOSAKI Motohiro <> | Subject | Re: [PATCH 1/2] SHM_UNLOCK: fix long unpreemptible section |
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(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@google.com> > Cc: stable@vger.kernel.org [back to 2.6.32 but will need respins]
Looks completely make sense. Reviewed-by: KOSAKI Motohiro <kosaki.motohiro@jp.fujitsu.com>
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