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

[Yosry Ahmed <yosryahmed@google.com>]

On Wed, Feb 8, 2023 at 9:45 AM Matthew Wilcox <willy@infradead.org> wrote:
>
> On Wed, Feb 08, 2023 at 08:01:01AM -0800, Luis Chamberlain wrote:
> > 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?
>
> I was thinking that the two calls to folio_add_lru() in mm/shmem.c
> should be conditional on sbinfo->noswap.
>

Wouldn't this cause the folio to not show up in any lru lists, even
the unevictable one, which may be a strange discrepancy?

Perhaps we can do something like shmem_lock(), which calls
mapping_set_unevictable(), which will make folio_evictable() return
true and the LRUs code will take care of the rest?