linux-kernel - Re: [PATCH v7] zswap: replace RB tree with xarray

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Message-ID: <ZfqPK7AVunq2SC1l@google.com>
Date: Wed, 20 Mar 2024 07:24:27 +0000
From: Yosry Ahmed <yosryahmed@...gle.com>
To: Chris Li <chrisl@...nel.org>
Cc: Andrew Morton <akpm@...ux-foundation.org>, linux-kernel@...r.kernel.org, 
	linux-mm@...ck.org, Nhat Pham <nphamcs@...il.com>, 
	Johannes Weiner <hannes@...xchg.org>, "Matthew Wilcox (Oracle)" <willy@...radead.org>, 
	Chengming Zhou <zhouchengming@...edance.com>, Barry Song <v-songbaohua@...o.com>
Subject: Re: [PATCH v7] zswap: replace RB tree with xarray

[..]
> > > -     /* map */
> > > -     spin_lock(&tree->lock);
> > >       /*
> > > -      * The folio may have been dirtied again, invalidate the
> > > -      * possibly stale entry before inserting the new entry.
> > > +      * We finish initializing the entry while it's already in xarray.
> > > +      * This is safe because:
> > > +      *
> > > +      * 1. Concurrent stores and invalidations are excluded by folio lock.
> > > +      *
> > > +      * 2. Writeback is excluded by the entry not being on the LRU yet.
> > > +      *    The publishing order matters to prevent writeback from seeing
> > > +      *    an incoherent entry.
> >
> > As I mentioned before, writeback is also protected by the folio lock.
> > Concurrent writeback will find the folio in the swapcache and abort. The
> > fact that the entry is not on the LRU yet is just additional protection,
> > so I don't think the publishing order actually matters here. Right?
> 
> Right. This comment is explaining why this publishing order does not
> matter. I think we are talking about the same thing here?

The comment literally says "the publishing order matters.." :)

I believe Johannes meant that we should only publish the entry to the
LRU once it is fully initialized, to prevent writeback from using a
partially initialized entry.

What I am saying is that, even if we add a partially initialized entry
to the zswap LRU, writeback will skip it anyway because the folio is
locked in the swapcache.

So basically I think the comment should say:

	/*
	 * We finish initializing the entry while it's already in the
	 * xarray. This is safe because the folio is locked in the swap
	 * cache, which should protect against concurrent stores,
	 * invalidations, and writeback.
	 */

Johannes, what do you think?