linux-kernel - Re: [PATCH 2/2] mm: zswap: remove unnecessary tree cleanups in zswap

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Date: Thu, 25 Jan 2024 01:03:19 -0800
From: Yosry Ahmed <yosryahmed@...gle.com>
To: Chengming Zhou <zhouchengming@...edance.com>
Cc: Johannes Weiner <hannes@...xchg.org>, Andrew Morton <akpm@...ux-foundation.org>, 
	Nhat Pham <nphamcs@...il.com>, Chris Li <chrisl@...nel.org>, Huang Ying <ying.huang@...el.com>, 
	linux-mm@...ck.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH 2/2] mm: zswap: remove unnecessary tree cleanups in zswap_swapoff()

> >>>> The second difference is the handling of lru entry, which is easy that we
> >>>> just zswap_lru_del() in tree lock.
> >>>
> >>> Why do we need zswap_lru_del() at all? We should have already isolated
> >>> the entry at that point IIUC.
> >>
> >> I was thinking how to handle the "zswap_lru_putback()" if not writeback,
> >> in which case we can't use the entry actually since we haven't got reference
> >> of it. So we can don't isolate at the entry, and only zswap_lru_del() when
> >> we are going to writeback actually.
> >
> > Why not just call zswap_lru_putback() before we unlock the folio?
>
> When early return because __read_swap_cache_async() return NULL or !folio_was_allocated,
> we don't have a locked folio yet. The entry maybe invalidated and freed concurrently.

Oh, that path, right.

If we don't isolate the entry straightaway, concurrent reclaimers will
see the same entry, call __read_swap_cache_async(), find the folio
already in the swapcache and stop shrinking. This is because usually
this means we are racing with swapin and hitting the warmer part of
the zswap LRU.

I am not sure if this would matter in practice, maybe Nhat knows
better. Perhaps we can rotate the entry in the LRU before calling
__read_swap_cache_async() to minimize the chances of such a race? Or
we can serialize the calls to __read_swap_cache_async() but this may
be an overkill.