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DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=linux.dev; s=key1; t=1759772652; h=from:from:reply-to:subject:subject:date:date:message-id:message-id: to:to:cc:cc:mime-version:mime-version:content-type:content-type: in-reply-to:in-reply-to:references:references; bh=QcosD5rlzA4TE7AntBZPfeAW0/FnSxIJ3VMLbX62WUE=; b=ZxqVh7ItL0Tel/7B30RzqqAsLAe9Tay1fWGS1l5VTeYoHM2MLnWp5pZaEFzgsOB+TSCK1s Ny80HiVfF9LMmEhag8nyFNiq/lRp0oFmHHyA4xV+enn3M8SmbIoUmrZ1ni8B3iabRROqIu 1TW2ECx5W1zO0B8tZwGI3KQxozwH1cw= From: Roman Gushchin To: Jan Kara Cc: Andrew Morton , linux-kernel@vger.kernel.org, "Matthew Wilcox (Oracle)" , Dev Jain , linux-mm@kvack.org Subject: Re: [PATCH v2] mm: readahead: make thp readahead conditional to mmap_miss logic In-Reply-To: (Jan Kara's message of "Mon, 6 Oct 2025 14:31:06 +0200") References: <20251006015409.342697-1-roman.gushchin@linux.dev> Date: Mon, 06 Oct 2025 10:44:07 -0700 Message-ID: <87qzvg6i3c.fsf@linux.dev> MIME-Version: 1.0 Content-Type: text/plain X-Migadu-Flow: FLOW_OUT X-Stat-Signature: 4iri74zyp834s8j8q74q75tbf7df6mrd X-Rspamd-Queue-Id: 67067A000A X-Rspamd-Server: rspam06 X-Rspam-User: X-HE-Tag: 1759772654-516648 X-HE-Meta: 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 g4r5LYYo JGvZw6t1iL2IKR2v0XJ4rxm7iDPf3JQ6v2Esq8vy3TBNZ1SuNXXyCq8eMGgiIpkT7RugTQw7fOJAza6UCj+h7w8OO/Y6UFkxDRpOak8Ks7N7u4VewIfqf21PjU9XoxEPp3RtblIJgu7DH0kQ4ARZSPV4nWzKkK8CS1scsXFAeaZ83LhKsYIvnFh0qz0BvjamWJH72soc92bohnE7M24TCpO1xhmG2vk3y3t2ZOn+6nFkuaFg1X1ynyJC700qhBDF01wtvG+SY7PceqnBaifUZb/+SuxYP4f00i11M8UKrxFG/1josneBvsqvZLRp4JGfzm4ZlIHBFKvTr7SRwQzNICii6epOsbkHlFFlfyr+s/bV+nnG6Blf4UjX3SEC9Jw/U2lHF 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: Jan Kara writes: > On Sun 05-10-25 18:54:09, Roman Gushchin wrote: >> Commit 4687fdbb805a ("mm/filemap: Support VM_HUGEPAGE for file mappings") >> introduced a special handling for VM_HUGEPAGE mappings: even if the >> readahead is disabled, 1 or 2 HPAGE_PMD_ORDER pages are >> allocated. >> >> This change causes a significant regression for containers with a >> tight memory.max limit, if VM_HUGEPAGE is widely used. Prior to this >> commit, mmap_miss logic would eventually lead to the readahead >> disablement, effectively reducing the memory pressure in the >> cgroup. With this change the kernel is trying to allocate 1-2 huge >> pages for each fault, no matter if these pages are used or not >> before being evicted, increasing the memory pressure multi-fold. >> >> To fix the regression, let's make the new VM_HUGEPAGE conditional >> to the mmap_miss check, but keep independent from the ra->ra_pages. >> This way the main intention of commit 4687fdbb805a ("mm/filemap: >> Support VM_HUGEPAGE for file mappings") stays intact, but the >> regression is resolved. >> >> The logic behind this changes is simple: even if a user explicitly >> requests using huge pages to back the file mapping (using VM_HUGEPAGE >> flag), under a very strong memory pressure it's better to fall back >> to ordinary pages. >> >> Signed-off-by: Roman Gushchin >> Reviewed-by: Jan Kara >> Cc: Matthew Wilcox (Oracle) >> Cc: Dev Jain >> Cc: linux-mm@kvack.org >> >> -- >> >> v2: fixed VM_SEQ_READ handling (by Dev Jain) > > OK, but now we'll do mmap_miss detection and bail-out even for VM_SEQ_READ > | VM_HUGEPAGE vmas. And without VM_HUGEPAGE we won't do it which is really > odd. So I think you want to make the whole mmap_miss logic conditional on > !VM_SEQ_READ... Yeah, agree, good point. Thanks!