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Howlett" , Mike Rapoport , Shakeel Butt , linux-mm@kvack.org, linux-kernel@vger.kernel.org, stable@vger.kernel.org Subject: Re: [PATCH v2 4/4] mm: page_alloc: fix non-movable reclaim storm in defrag_mode Message-ID: References: <20260722150006.3848560-1-hannes@cmpxchg.org> <20260722150006.3848560-5-hannes@cmpxchg.org> <20260722162207.31a2c491ffaf36d1e8948c14@linux-foundation.org> <67e1219e-9d47-4a30-a8dd-fdcf03229f58@kernel.org> Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <67e1219e-9d47-4a30-a8dd-fdcf03229f58@kernel.org> On Thu, Jul 23, 2026 at 12:22:58PM +0200, Vlastimil Babka (SUSE) wrote: > + Greg and Sasha > > On 7/23/26 01:22, Andrew Morton wrote: > > On Wed, 22 Jul 2026 10:56:47 -0400 Johannes Weiner wrote: > > > >> > >> Fixes: e3aa7df331bc ("mm: page_alloc: defrag_mode") > >> Cc: > >> Signed-off-by: Johannes Weiner > > > > This cc:stable fix doesn't apply to current mainline because the > > preceding non-cc:stable patches get in the way. > > > > This means that the -stable maintainers will have to ask you to redo > > the fix to make it backportable. And you'll then be preparing a patch > > which hasn't had standalone testing in mainline. All very cumbersome. > > Or they backport the whole series, even if only 4/4 has a Fixes: tag. > > > So is it practical to make the fix come first, as a cc:stable hotfix so > > we can upstream the other patches in the next merge window? > > I think it can't come first, it's simply a complex fix that can't be done in > a single patch that would be reviewable. 3/4 is certainly a strict > dependency. I'm not sure if 2/4 as well. Perhaps 2/4 does deserve a Fixes: > tag and Cc: stable on its own, as it also fixes a defrag_mode deficiency. Yes. Only 1/4 could be argued away. 2/4 actually came after 4/4 during development. Without it, compaction cannot make sufficient forward progress on the promoted non-movable requests. So it is a hard functional requirement. Whether it deserves an independent Fixes: depends on how many GFP_KERNEL pageblock_order requests we have :-) I'm guessing not many. > > (An alternative would be to put cc:stable on all four patches. That's > > a bit abusive but we do it rarely). > > I don't think it's necessary, the stable team are quite capable of figuring > out the deps and backporting them with a Stable-dep-off: tag. IIRC the > "fake" tagging is discouraged? Maybe they can clarify. Yes, I *think* it should work with tagging just 4/4. It won't apply cleanly or build without 3/4. So I'd assume if that dependency is lost, the automated backport will bounce and I can send a backport series by hand.