linux-kernel - Re: [PATCH v6 6/6] zsmalloc: Implement writeback mechanism for zsmalloc

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Date:   Tue, 22 Nov 2022 01:09:03 -0500
From:   Johannes Weiner <hannes@...xchg.org>
To:     Sergey Senozhatsky <senozhatsky@...omium.org>
Cc:     Nhat Pham <nphamcs@...il.com>, akpm@...ux-foundation.org,
        linux-mm@...ck.org, linux-kernel@...r.kernel.org,
        minchan@...nel.org, ngupta@...are.org, sjenning@...hat.com,
        ddstreet@...e.org, vitaly.wool@...sulko.com
Subject: Re: [PATCH v6 6/6] zsmalloc: Implement writeback mechanism for
 zsmalloc

On Tue, Nov 22, 2022 at 12:42:20PM +0900, Sergey Senozhatsky wrote:
> On (22/11/21 22:12), Johannes Weiner wrote:
> > On Tue, Nov 22, 2022 at 11:15:20AM +0900, Sergey Senozhatsky wrote:
> > > On (22/11/18 16:15), Nhat Pham wrote:
> > > > +
> > > > +static int zs_zpool_shrink(void *pool, unsigned int pages,
> > > > +			unsigned int *reclaimed)
> > > > +{
> > > > +	unsigned int total = 0;
> > > > +	int ret = -EINVAL;
> > > > +
> > > > +	while (total < pages) {
> > > > +		ret = zs_reclaim_page(pool, 8);
> > > > +		if (ret < 0)
> > > > +			break;
> > > > +		total++;
> > > > +	}
> > > > +
> > > > +	if (reclaimed)
> > > > +		*reclaimed = total;
> > > > +
> > > > +	return ret;
> > > > +}
> > > 
> > > A silly question: why do we need a retry loop in zs_reclaim_page()?
> > 
> > Individual objects in a zspage can be busy (swapped in simultaneously
> > for example), which will prevent the zspage from being freed. Zswap
> > currently requests reclaim of one backend page at a time (another
> > project...), so if we don't retry we're not meeting the reclaim goal
> > and cause rejections for new stores.
> 
> What I meant was: if zs_reclaim_page() makes only partial progress
> with the current LRU tail zspage and returns -EAGAIN, then we just
> don't increment `total` and continue looping in zs_zpool_shrink().

Hm, but it breaks on -EAGAIN, it doesn't continue.

This makes sense, IMO. zs_reclaim_page() will try to reclaim one page,
but considers up to 8 LRU tail pages until one succeeds. If it does,
it continues (total++). But if one of these calls fails, we exit the
loop, give up and return failure from zs_zpool_shrink().