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b=LTnS4nix+lkxGZFdMT9owY8sNAP2Fm00l2wZiWUZaMK0WnBxMYzlEkKLDz8j741ax 1jTn66jrsywkmyeDPoAedANWTuhkmMGYyMpfyAvVuQMbICNiD5gkvNPhAlBDM1WPob ggnU7wh3w4MhajY559HTtkxyMOOk+JbV0RduD6OBBvSgl7bocO4W/0MCmbAn6/hweq qK7mBz9qxZEZj4qv4Q97QUiRMqGCn2Dp0S02CLG/SuujwoKVfPe/Bd9+a1ws6OqD7p Nn7Yv6ttUysNOgQ34XY0CZFuFalBS0IxW4d2MNMP3pVE8z0HJpPhAFJq5EtmIe4dUF NnJXShK2uI2fg== Date: Wed, 2 Sep 2026 12:27:59 +0100 From: Harry Yoo To: Hyunwoo Kim Cc: "Vlastimil Babka (SUSE)" , Hao Li , Andrew Morton , Christoph Lameter , David Rientjes , Roman Gushchin , Suren Baghdasaryan , linux-mm@kvack.org, linux-kernel@vger.kernel.org Subject: Re: [PATCH] mm/slab: take n->list_lock for the list_add() in __refill_objects_node() Message-ID: References: MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: X-Stat-Signature: mthq5mn4m73wiq9zcenh5dfp768e6k3d X-Rspamd-Queue-Id: 39E79A000B X-Rspamd-Server: rspam02 X-Rspam-User: X-HE-Tag: 1788348482-657407 X-HE-Meta: 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 Qcw9qad2 SZ53kZaH6LPSOTQB2lpM08wyq0woZ26rq2zPRxU5errEm89/MGXLL8BciU8jGvZoYq6zKiU52etQ6sTgOgGb/PX5v3zYvy9Cp2EUZu9CdZ38BVHpuKHVmDyYPyNAMuLSIBAtF1KDW+gzvAsAT+q4l8CN2cY1mp+1/1r2crgcde6foWvWXRRdgwO85fY5qdf/LcdavY5ldT6C668HlZaprwzS5R4lI9kSi1jGQ3/amENItXy9PeStA04NJ4rb1oy1g5gPcK5N5ljaajXzIsT6KdMXtSRN4AbUFc1SbjGSInPnHlznaIZjv4ED1B200g/uvMJYI Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: List-Subscribe: List-Unsubscribe: On Wed, Sep 02, 2026 at 08:24:27PM +0900, Hyunwoo Kim wrote: > On Wed, Sep 02, 2026 at 12:20:34PM +0100, Harry Yoo wrote: > > On Mon, Aug 31, 2026 at 02:55:06PM +0200, Vlastimil Babka (SUSE) wrote: > > > On 8/30/26 16:35, Hao Li wrote: > > > > On Sun, Aug 30, 2026 at 12:45:17PM +0000, Harry Yoo wrote: > > > >> On Sun, Aug 30, 2026 at 04:25:45PM +0900, Hyunwoo Kim wrote: > > > > Since introducing a new variable seems unavoidable, what if we temporarily > > > > stash this slab in a pointer like below, and then add it to pc.slabs once we > > > > acquire the lock. > > > > > > > > struct slab *leftover_slab = NULL; > > > > > > > > ... > > > > ... > > > > if (__slab_try_return_freelist(s, slab, head, count)) { > > > > leftover_slab = slab; > > > > break; > > > > } > > > > > > > > ... > > > > ... > > > > if (!list_empty(&pc.slabs)) { > > > > spin_lock_irqsave(&n->list_lock, flags); > > > > > > > > if (leftover_slab) > > > > list_add(&leftover_slab->slab_list, &pc.slabs); > > > > ... > > > > ... > > > > } > > > > > > > > PS: If I recall correctly, Vlastimil's initial patch was actually fine. It was > > > > my suggestion to save an extra lock/unlock pair that accidentally led to this > > > > trap... > > > > > > Ah, thanks for the reminder. This [1] was the original attempt. > > > > > > [1] > > > https://lore.kernel.org/all/20260421-b4-refill-optimistic-return-v1-1-24f0bfc1acff@kernel.org/ > > > > > > I wonder if the fix should be to return to that approach and just have > > > __slab_try_return_freelist() handle the list_lock. The code would be simpler > > > with not "bool locked". > > > > > > It should be really very rare that we would end up returning a partial list > > > and also have additional slabs to return on pc.slabs? So I think there would > > > be no noticeable performance downside to the simpler code potentially ending > > > up taking the list_lock twice instead of once. > > > > Agreed that it should be rare and not worth the complexity unless > > we have data to support that. > > > > Hyunwoo, would you please adjust the feedback and post v2? > > If you don't mind, could the maintainers handle this patch on your side? > I'd then move to Reported-by:. I'll post v2 with your Reported-by: then. Thanks! > Best regards, > Hyunwoo Kim -- Cheers, Harry / Hyeonggon