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[70.44.39.90]) by smtp.gmail.com with ESMTPSA id r2sm6445507qti.4.2021.02.19.16.34.29 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Fri, 19 Feb 2021 16:34:29 -0800 (PST) Date: Fri, 19 Feb 2021 19:34:29 -0500 From: Johannes Weiner To: Shakeel Butt Cc: Hugh Dickins , Roman Gushchin , Michal Hocko , Andrew Morton , cgroups@vger.kernel.org, linux-mm@kvack.org, linux-kernel@vger.kernel.org Subject: Re: [PATCH] memcg: charge before adding to swapcache on swapin Message-ID: References: <20210219224405.1544597-1-shakeelb@google.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20210219224405.1544597-1-shakeelb@google.com> Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Fri, Feb 19, 2021 at 02:44:05PM -0800, Shakeel Butt wrote: > Currently the kernel adds the page, allocated for swapin, to the > swapcache before charging the page. This is fine but now we want a > per-memcg swapcache stat which is essential for folks who wants to > transparently migrate from cgroup v1's memsw to cgroup v2's memory and > swap counters. > > To correctly maintain the per-memcg swapcache stat, one option which > this patch has adopted is to charge the page before adding it to > swapcache. One challenge in this option is the failure case of > add_to_swap_cache() on which we need to undo the mem_cgroup_charge(). > Specifically undoing mem_cgroup_uncharge_swap() is not simple. > > This patch circumvent this specific issue by removing the failure path > of add_to_swap_cache() by providing __GFP_NOFAIL. Please note that in > this specific situation ENOMEM was the only possible failure of > add_to_swap_cache() which is removed by using __GFP_NOFAIL. > > Another option was to use __mod_memcg_lruvec_state(NR_SWAPCACHE) in > mem_cgroup_charge() but then we need to take of the do_swap_page() case > where synchronous swap devices bypass the swapcache. The do_swap_page() > already does hackery to set and reset PageSwapCache bit to make > mem_cgroup_charge() execute the swap accounting code and then we would > need to add additional parameter to tell to not touch NR_SWAPCACHE stat > as that code patch bypass swapcache. > > This patch added memcg charging API explicitly foe swapin pages and > cleaned up do_swap_page() to not set and reset PageSwapCache bit. > > Signed-off-by: Shakeel Butt The patch makes sense to me. While it extends the charge interface, I actually quite like that it charges the page earlier - before putting it into wider circulation. It's a step in the right direction. But IMO the semantics of mem_cgroup_charge_swapin_page() are a bit too fickle: the __GFP_NOFAIL in add_to_swap_cache() works around it, but having a must-not-fail-after-this line makes the code tricky to work on and error prone. It would be nicer to do a proper transaction sequence. > @@ -497,16 +497,15 @@ struct page *__read_swap_cache_async(swp_entry_t entry, gfp_t gfp_mask, > __SetPageLocked(page); > __SetPageSwapBacked(page); > > - /* May fail (-ENOMEM) if XArray node allocation failed. */ > - if (add_to_swap_cache(page, entry, gfp_mask & GFP_RECLAIM_MASK, &shadow)) { > - put_swap_page(page, entry); > + if (mem_cgroup_charge_swapin_page(page, NULL, gfp_mask, entry)) > goto fail_unlock; > - } > > - if (mem_cgroup_charge(page, NULL, gfp_mask)) { > - delete_from_swap_cache(page); > - goto fail_unlock; > - } > + /* > + * Use __GFP_NOFAIL to not worry about undoing the changes done by > + * mem_cgroup_charge_swapin_page() on failure of add_to_swap_cache(). > + */ > + add_to_swap_cache(page, entry, > + (gfp_mask|__GFP_NOFAIL) & GFP_RECLAIM_MASK, &shadow); How about: mem_cgroup_charge_swapin_page() add_to_swap_cache() mem_cgroup_finish_swapin_page() where finish_swapin_page() only uncharges the swap entry (on cgroup1) once the swap->memory transition is complete? Otherwise the patch looks good to me.