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From: Shakeel Butt To: Yosry Ahmed Cc: Usama Arif , akpm@linux-foundation.org, willy@infradead.org, hannes@cmpxchg.org, nphamcs@gmail.com, chengming.zhou@linux.dev, linux-mm@kvack.org, linux-kernel@vger.kernel.org, kernel-team@meta.com Subject: Re: [PATCH] mm: Do not start/end writeback for pages stored in zswap Message-ID: References: <20240610143037.812955-1-usamaarif642@gmail.com> MIME-Version: 1.0 Content-Type: text/plain; charset=utf-8 Content-Disposition: inline Content-Transfer-Encoding: 8bit In-Reply-To: X-Migadu-Flow: FLOW_OUT X-Rspamd-Server: rspam07 X-Rspamd-Queue-Id: 851A1C0019 X-Stat-Signature: bm9mzub7c7uxr4u38y6sonjyp5wq4wob X-Rspam-User: X-HE-Tag: 1718046496-404665 X-HE-Meta: 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 Rin0MkQW 211YvWBVbf+k2XDWg7lWUjF379GQzUm9XPUFhBX3FlqkaDmM9y5z8yR/kfxMzFREYXW0mbjuvdcYV/YR/X23LNOdvY4v/M8Ww5JsEWCbcD9QG7BS4NByqVVsZeALYIad40Aa4yhD3FLc7EznEkYD7Asovag9hWZ4kyoDrR7VOffjBwTvClgD7dv8ZxRtVtc9xsqrqqVyatWMMrtE= X-Bogosity: Ham, tests=bogofilter, spamicity=0.000000, version=1.2.4 Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: List-Subscribe: List-Unsubscribe: On Mon, Jun 10, 2024 at 10:31:36AM GMT, Yosry Ahmed wrote: > On Mon, Jun 10, 2024 at 7:31 AM Usama Arif wrote: > > > > start/end writeback combination incorrectly increments NR_WRITTEN > > counter, eventhough the pages aren't written to disk. Pages successfully > > stored in zswap should just unlock folio and return from writepage. > > > > Signed-off-by: Usama Arif > > --- > > mm/page_io.c | 2 -- > > 1 file changed, 2 deletions(-) > > > > diff --git a/mm/page_io.c b/mm/page_io.c > > index a360857cf75d..501784d79977 100644 > > --- a/mm/page_io.c > > +++ b/mm/page_io.c > > @@ -196,9 +196,7 @@ int swap_writepage(struct page *page, struct writeback_control *wbc) > > return ret; > > } > > if (zswap_store(folio)) { > > - folio_start_writeback(folio); > > folio_unlock(folio); > > - folio_end_writeback(folio); > > Removing these calls will have several effects, I am not really sure it's safe. > > 1. As you note in the commit log, NR_WRITTEN stats (and apparently > others) will no longer be updated. While this may make sense, it's a > user-visible change. I am not sure if anyone relies on this. > I couldn't imagine how this stat can be useful for the zswap case and I don't see much risk in changing this stat behavior for such cases. > 2. folio_end_writeback() calls folio_rotate_reclaimable() after > writeback completes to put a folio that has been marked with > PG_reclaim at the tail of the LRU, to be reclaimed first next time. Do > we get this call through other paths now? > The folio_rotate_reclaimable() only makes sense for async writeback pages i.e. not for zswap where we synchronously reclaim the page. > 3. If I remember correctly, there was some sort of state machine where > folios go from dirty to writeback to clean. I am not sure what happens > if we take the writeback phase out of the equation. > Is there really such a state machine? We only trigger writeback if the page is dirty and we have cleared it. The only thing I can think of is the behavior of the waiters on PG_locked bit but the window of PG_writeback is so small that it seems like it does not matter.