linux-kernel - Re: [RFC 2/2] shmem: add support to ignore swap

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Date:   Wed, 8 Feb 2023 08:01:01 -0800
From:   Luis Chamberlain <mcgrof@...nel.org>
To:     Matthew Wilcox <willy@...radead.org>
Cc:     hughd@...gle.com, akpm@...ux-foundation.org, linux-mm@...ck.org,
        p.raghav@...sung.com, dave@...olabs.net, a.manzanares@...sung.com,
        linux-kernel@...r.kernel.org
Subject: Re: [RFC 2/2] shmem: add support to ignore swap

On Tue, Feb 07, 2023 at 04:01:51AM +0000, Matthew Wilcox wrote:
> On Mon, Feb 06, 2023 at 06:52:59PM -0800, Luis Chamberlain wrote:
> > @@ -1334,11 +1336,15 @@ static int shmem_writepage(struct page *page, struct writeback_control *wbc)
> >  	struct shmem_inode_info *info;
> >  	struct address_space *mapping = folio->mapping;
> >  	struct inode *inode = mapping->host;
> > +	struct shmem_sb_info *sbinfo = SHMEM_SB(inode->i_sb);
> >  	swp_entry_t swap;
> >  	pgoff_t index;
> >  
> >  	BUG_ON(!folio_test_locked(folio));
> >  
> > +	if (wbc->for_reclaim && unlikely(sbinfo->noswap))
> > +		return AOP_WRITEPAGE_ACTIVATE;
> 
> Not sure this is the best way to handle this.  We'll still incur the
> oevrhead of tracking shmem pages on the LRU, only to fail to write them
> out when the VM thinks we should get rid of them.  We'd be better off
> not putting them on the LRU in the first place.

Ah, makes sense, so in effect then if we do that then on reclaim
we should be able to even WARN_ON(sbinfo->noswap) assuming we did
everthing right.

Hrm, we have invalidate_mapping_pages(mapping, 0, -1) but that seems a bit
too late how about d_mark_dontcache() on shmem_get_inode() instead?

  Luis