Received-SPF: pass (google.com: domain of linux-kernel+bounces-108732-linux.lists.archive=gmail.com@vger.kernel.org designates 139.178.88.99 as permitted sender) client-ip=139.178.88.99;
Date: Wed, 20 Mar 2024 06:08:03 -0400
From: Johannes Weiner <hannes@cmpxchg.org>
To: Yosry Ahmed <yosryahmed@google.com>
Cc: Chris Li <chrisl@kernel.org>, Andrew Morton <akpm@linux-foundation.org>,
	linux-kernel@vger.kernel.org, linux-mm@kvack.org,
	Nhat Pham <nphamcs@gmail.com>,
	"Matthew Wilcox (Oracle)" <willy@infradead.org>,
	Chengming Zhou <zhouchengming@bytedance.com>,
	Barry Song <v-songbaohua@oppo.com>
Subject: Re: [PATCH v7] zswap: replace RB tree with xarray
Message-ID: <20240320100803.GB294822@cmpxchg.org>
References: <20240319-zswap-xarray-v7-1-e9a03a049e86@kernel.org>
 <Zfp-iWaDfqeCOElt@google.com>
 <CANeU7Q=yxf0dnerTOZfe_ioeCbjnZd2Fpb-szvW7-Q1BzCUpOw@mail.gmail.com>
 <ZfqPK7AVunq2SC1l@google.com>
Precedence: bulk
MIME-Version: 1.0
Content-Type: text/plain; charset=us-ascii
Content-Disposition: inline
In-Reply-To: <ZfqPK7AVunq2SC1l@google.com>

On Wed, Mar 20, 2024 at 07:24:27AM +0000, Yosry Ahmed wrote:
> [..]
> > > > -     /* 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?

I don't think that's quite right.

Writeback will bail on swapcache insert, yes, but it will access the
entry before attempting it. If LRU publishing happened before setting
entry->swpentry e.g., we'd have a problem, while your comment suggets
it would be safe to rearrange the code like this.

So LRU publishing order does matter.