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b=UgfuzvJATgMXu9jg/0qc26PjyHiJisgzgmjJPG1ueIoegr8R6ylVc+dRzgCChHDaS ryFJFpTy5dZjVSmbEo2qN1YrW85+TCE2kYy8eVb2zz633m7Pt15GdTegVnEuC0KWq1 O4AjOtDEIFUxnaCORLbKBJAp+G9RNz6xsMp4UB9E= Date: Thu, 27 Jun 2024 16:32:42 -0700 From: Andrew Morton To: Peter Xu Cc: Yang Shi , yangge1116@126.com, david@redhat.com, linux-mm@kvack.org, linux-kernel@vger.kernel.org, stable@vger.kernel.org Subject: Re: [v2 PATCH] mm: gup: do not call try_grab_folio() in slow path Message-Id: <20240627163242.39b0a716bd950a895c032136@linux-foundation.org> In-Reply-To: References: <20240627221413.671680-1-yang@os.amperecomputing.com> X-Mailer: Sylpheed 3.8.0beta1 (GTK+ 2.24.33; x86_64-pc-linux-gnu) Mime-Version: 1.0 Content-Type: text/plain; charset=US-ASCII Content-Transfer-Encoding: 7bit X-Rspamd-Queue-Id: CC3DE14001C X-Rspam-User: X-Rspamd-Server: rspam05 X-Stat-Signature: mc5jgh7fyekn478fe7di5mjswjjx8k9x X-HE-Tag: 1719531164-22248 X-HE-Meta: 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 PDNvYU+r Ejt1BVvdfvh+joRhub7N/DBEGH6yZhQ6An7TakYwdMJLwPM5nG5VsBbjNBcfvdo0e4nNI3QPeUIHnfAcbQJuLHYLgcypKl+A7Us5hMXRUkgNoGchA3vM/L5nxgeWYTZpuDzEIQfonCTfqED81yNGof5bX4LhLbi8hrhNWDHKiv0MSvnFsdyolpUTxBJNUN91wwbzxx6BiyWMJSwd8YQwWvggniIhOBYqqpK3CNZX+Y113B+6nY8Wd11pyTwJ+4Lds9w8zAtykQDpknfmWs2FJmpBXpWiLvVKgpFZStybtWimZr/g= 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 Thu, 27 Jun 2024 19:19:40 -0400 Peter Xu wrote: > Yang, > > On Thu, Jun 27, 2024 at 03:14:13PM -0700, Yang Shi wrote: > > The try_grab_folio() is supposed to be used in fast path and it elevates > > folio refcount by using add ref unless zero. We are guaranteed to have > > at least one stable reference in slow path, so the simple atomic add > > could be used. The performance difference should be trivial, but the > > misuse may be confusing and misleading. > > This first paragraph is IMHO misleading itself.. > > I think we should mention upfront the important bit, on the user impact. > > Here IMO the user impact should be: Linux may fail longterm pin in some > releavnt paths when applied over CMA reserved blocks. And if to extend a > bit, that include not only slow-gup but also the new memfd pinning, because > both of them used try_grab_folio() which used to be only for fast-gup. It's still unclear how users will be affected. What do the *users* see? If it's a slight slowdown, do we need to backport this at all? > > The patch itself looks mostly ok to me. > > There's still some "cleanup" part mangled together, e.g., the real meat > should be avoiding the folio_is_longterm_pinnable() check in relevant > paths. The rest (e.g. switch slow-gup / memfd pin to use folio_ref_add() > not try_get_folio(), and renames) could be good cleanups. > > So a smaller fix might be doable, but again I don't have a strong opinion > here. The smaller the better for backporting, of course.