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SubjectRe: [PATCH 2/2] sysvshm: SHM_LOCK use lru_add_drain_all_async()
On Sun, 1 Jan 2012, kosaki.motohiro@gmail.com wrote:
> From: KOSAKI Motohiro <kosaki.motohiro@jp.fujitsu.com>
>
> shmctl also don't need synchrounous pagevec drain. This patch replace it with
> lru_add_drain_all_async().
>
> Signed-off-by: KOSAKI Motohiro <kosaki.motohiro@jp.fujitsu.com>

Let me answer this 2/2 first since it's easier.

I'm going to thank you for bringing this lru_add_drain_all()
to my attention, I had not noticed it; but Nak the patch itself.

The reason being, that particular lru_add_drain_all() serves no
useful purpose, so let's delete it instead of replacing it. I believe
that it serves no purpose for SHM_LOCK and no purpose for SHM_UNLOCK.

I'm dabbling in this area myself, since you so cogently pointed out that
I'd tried to add a cond_resched() to scan_mapping_unevictable_pages()
(which is a helper for SHM_UNLOCK here) while it's under spinlock.

In testing my fix for that, I find that there has been no attempt to
keep the Unevictable count accurate on SysVShm: SHM_LOCK pages get
marked unevictable lazily later as memory pressure discovers them -
which perhaps mirrors the way in which SHM_LOCK makes no attempt to
instantiate pages, unlike mlock.

Since nobody has complained about that in the two years since we've
had an Unevictable count in /proc/meminfo, I don't see any need to
add code (it would need more than just your change here; would need
more even than calling scan_mapping_unevictable_pages() at SHM_LOCK
time - though perhaps along with your 1/2 that could handle it) and
overhead to satisfy a need that nobody has.

I'll delete that lru_add_drain_all() in my patch, okay?

(But in writing this, realize I still don't quite understand why
the Unevictable count takes a second or two to get back to 0 after
SHM_UNLOCK: perhaps I've more to discover.)

Hugh

> ---
> ipc/shm.c | 4 ++--
> 1 files changed, 2 insertions(+), 2 deletions(-)
>
> diff --git a/ipc/shm.c b/ipc/shm.c
> index 02ecf2c..1eb25f0 100644
> --- a/ipc/shm.c
> +++ b/ipc/shm.c
> @@ -872,8 +872,6 @@ SYSCALL_DEFINE3(shmctl, int, shmid, int, cmd, struct shmid_ds __user *, buf)
> {
> struct file *uninitialized_var(shm_file);
>
> - lru_add_drain_all(); /* drain pagevecs to lru lists */
> -
> shp = shm_lock_check(ns, shmid);
> if (IS_ERR(shp)) {
> err = PTR_ERR(shp);
> @@ -911,6 +909,8 @@ SYSCALL_DEFINE3(shmctl, int, shmid, int, cmd, struct shmid_ds __user *, buf)
> shp->mlock_user = NULL;
> }
> shm_unlock(shp);
> + /* prevent user visible mismatch of unevictable accounting */
> + lru_add_drain_all_async();
> goto out;
> }
> case IPC_RMID:
> --
> 1.7.1


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