Re: [PATCH 5/5] mm/zswap: cleanup zswap_reclaim_entry()

[Yosry Ahmed <yosryahmed@google.com>]

On Mon, Dec 18, 2023 at 6:58 AM Johannes Weiner <hannes@cmpxchg.org> wrote:
>
> On Mon, Dec 18, 2023 at 06:39:13AM -0800, Yosry Ahmed wrote:
> > On Mon, Dec 18, 2023 at 6:03 AM Johannes Weiner <hannes@cmpxchg.org> wrote:
> > >
> > > On Thu, Dec 14, 2023 at 02:41:26PM -0800, Yosry Ahmed wrote:
> > > > On Thu, Dec 14, 2023 at 2:23 PM Andrew Morton <akpm@linux-foundation.org> wrote:
> > > > >
> > > > > On Wed, 13 Dec 2023 17:02:25 -0800 Yosry Ahmed <yosryahmed@google.com> wrote:
> > > > >
> > > > > > On Tue, Dec 12, 2023 at 8:18 PM Chengming Zhou
> > > > > > <zhouchengming@bytedance.com> wrote:
> > > > > > >
> > > > > > > Also after the common decompress part goes to __zswap_load(), we can
> > > > > > > cleanup the zswap_reclaim_entry() a little.
> > > > > >
> > > > > > I think you mean zswap_writeback_entry(), same for the commit title.
> > > > >
> > > > > I updated my copy of the changelog, thanks.
> > > > >
> > > > > > > -       /*
> > > > > > > -        * If we get here because the page is already in swapcache, a
> > > > > > > -        * load may be happening concurrently. It is safe and okay to
> > > > > > > -        * not free the entry. It is also okay to return !0.
> > > > > > > -        */
> > > > > >
> > > > > > This comment should be moved above the failure check of
> > > > > > __read_swap_cache_async() above, not completely removed.
> > > > >
> > > > > This?
> > > >
> > > > Yes, thanks a lot. Although I think a new version is needed anyway to
> > > > address other comments.
> > > >
> > > > >
> > > > > --- a/mm/zswap.c~mm-zswap-cleanup-zswap_reclaim_entry-fix
> > > > > +++ a/mm/zswap.c
> > > > > @@ -1457,8 +1457,14 @@ static int zswap_writeback_entry(struct
> > > > >         mpol = get_task_policy(current);
> > > > >         page = __read_swap_cache_async(swpentry, GFP_KERNEL, mpol,
> > > > >                                 NO_INTERLEAVE_INDEX, &page_was_allocated, true);
> > > > > -       if (!page)
> > > > > +       if (!page) {
> > > > > +               /*
> > > > > +                * If we get here because the page is already in swapcache, a
> > > > > +                * load may be happening concurrently. It is safe and okay to
> > > > > +                * not free the entry. It is also okay to return !0.
> > > > > +                */
> > > > >                 return -ENOMEM;
> > > > > +       }
> > > > >
> > > > >         /* Found an existing page, we raced with load/swapin */
> > > > >         if (!page_was_allocated) {
> > >
> > > That's the wrong branch, no?
> > >
> > > !page -> -ENOMEM
> > >
> > > page && !page_was_allocated -> already in swapcache
> >
> > Ah yes, my bad.
> >
> > >
> > > Personally, I don't really get the comment. What does it mean that
> > > it's "okay" not to free the entry? There is a put, which may or may
> > > not free the entry if somebody else is using it. Is it explaining how
> > > lifetime works for refcounted objects? I'm similarly confused by the
> > > "it's okay" to return non-zero. What is that trying to convey?
> > >
> > > Deletion seemed like the right choice here, IMO ;)
> >
> > It's not the clearest of comments for sure. I think it is just trying
> > to say that it is okay not to write back the entry from zswap and to
> > fail, because the caller will just try another page. I did not like
> > silently deleting the comment during the refactoring. How about
> > rewriting it to something like:
> >
> > /*
> >  * If we get here because the page is already in the swapcache, a
> >  * load may be happening concurrently. Skip this page, the caller
> >  * will move on to a different page.
> >  */
>
> Well there is this one already on the branch:
>
> /* Found an existing page, we raced with load/swapin */
>
> which covers the first half. The unspoken assumption there is that
> writeback is an operation for an aged out page, while swapin means the
> age just got reset to 0. Maybe it makes sense to elaborate on that?

How about the following diff? This applies on top of Andrew's fix:

diff --git a/mm/zswap.c b/mm/zswap.c
index e8f8f47596dae..8228a0b370979 100644
--- a/mm/zswap.c
+++ b/mm/zswap.c
@@ -1458,15 +1458,14 @@ static int zswap_writeback_entry(struct
zswap_entry *entry,
        page = __read_swap_cache_async(swpentry, GFP_KERNEL, mpol,
                                NO_INTERLEAVE_INDEX, &page_was_allocated, true);
        if (!page) {
-               /*
-                * If we get here because the page is already in swapcache, a
-                * load may be happening concurrently. It is safe and okay to
-                * not free the entry. It is also okay to return !0.
-                */
                return -ENOMEM;
        }

-       /* Found an existing page, we raced with load/swapin */
+       /*
+        * Found an existing page, we raced with load/swapin. We generally
+        * writeback cold pages from zswap, and swapin means the page just
+        * became hot. Skip this page and let the caller find another one.
+        */
        if (!page_was_allocated) {
                put_page(page);
                return -EEXIST;