From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from smtp.kernel.org (aws-us-west-2-korg-mail-alma10-1.taild15c8.ts.net [100.103.45.18]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by smtp.subspace.kernel.org (Postfix) with ESMTPS id 36CD44483B9 for ; Thu, 30 Jul 2026 15:28:39 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.client-ip=100.103.45.18 ARC-Seal:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1785425320; cv=none; b=Ur7ThxRpwfzCymmNqiY1tV1G4D2TPHS2R8Tr9QwTfunKpsfyo/pI48okewS9ATLLFQ302R++xmNyZF0vnxiO/zB0PtNg2B5UgKV6016pRatoVj3liVqXaG9oPRe121NOexHor1VxgFtVS85wENuTCmkeITQGUn2lgnabty4v22E= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1785425320; c=relaxed/simple; bh=bymkIqzlx8PCr2/y+Nlvlx6nzKZly6V6Mx7WMgbnlN8=; h=Message-ID:Date:MIME-Version:Subject:To:Cc:References:From: In-Reply-To:Content-Type; b=ef9+nm5nMO1In5ayGAQ81lAmv7fzn3ULUsfiPb/HZIsPxng2/OXhnssTkec2YDV3wBx2py5XLVFlJEscZEsjQ3W/boSo7/qE9gy0CO7vDsANOY2ndC39NGSlwQjoQ5gbnuY8hNMFnfQlFkX0NegpXwCZdB/mH3xO9bEGAkUtRvo= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=kernel.org header.i=@kernel.org header.b=Vy31z78X; arc=none smtp.client-ip=100.103.45.18 Authentication-Results: smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=kernel.org header.i=@kernel.org header.b="Vy31z78X" Received: by smtp.kernel.org (Postfix) with ESMTPSA id 2F78E1F000E9; Thu, 30 Jul 2026 15:28:38 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=kernel.org; s=k20260515; t=1785425319; bh=7aGN8weU+L7DqgVQRgWHsE0n2+CyhRki/zqPigzBOM0=; h=Date:Subject:To:Cc:References:From:In-Reply-To; b=Vy31z78XoTEU+GQPKvefB1jK45bLSI4i9veb1bxkb8p5FDQ0FA66fGUfxdhXJfPcP QtF3yyxjOVLk+IuSUe+8Iq0eG64ukv8xNHFbR5VXMX1yqpzmq3/yGgO7VIWA0wWgNO SEHdkzCwruqPIUjuK2GgcejNyk8AgfKHO3Jur7vdQkgx2KC+OTwHDY1IUyk+McSdsg MXuJ3sQ4+SV59X5e5bTdKQiVm9xwDw+1NGqT64Jfzh9J6plHtOxuChwnBr/g+rQvOZ 5xyKNubRoXOkkyX9qIzct3zJj8MIq2o2Orv9RBBAr9SVTkPkopjM/v5uqvpC8fsoNa 6jy97VPG+zwxw== Message-ID: <3105745c-f547-481c-a329-9b78ff4de0f8@kernel.org> Date: Thu, 30 Jul 2026 17:28:35 +0200 Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH] mm/page_alloc: only update lowmem_reserve_ratio on sysctl write Content-Language: en-US To: Jianlin Shi , linux-mm@kvack.org Cc: akpm@linux-foundation.org, linux-kernel@vger.kernel.org References: From: "Vlastimil Babka (SUSE)" Autocrypt: addr=vbabka@kernel.org; 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charset=UTF-8 Content-Transfer-Encoding: 7bit On 7/28/26 12:32, Jianlin Shi wrote: > lowmem_reserve_ratio_sysctl_handler() ignores the return value of > proc_dointvec_minmax() and always sanitizes sysctl_lowmem_reserve_ratio > and calls setup_per_zone_lowmem_reserve(), even for read operations. > > Reading /proc/sys/vm/lowmem_reserve_ratio should not recompute per-zone > lowmem_reserve[] and totalreserve_pages. Only do so when the sysctl is > written, matching min_free_kbytes and watermark_scale_factor handlers. > > Also propagate errors from proc_dointvec_minmax() instead of ignoring > them. > > Compatibility note: > Previously a read also sanitized sysctl_lowmem_reserve_ratio[] and > called setup_per_zone_lowmem_reserve(), which rewrites each zone's > lowmem_reserve[] and recalculates pgdat->totalreserve_pages / > totalreserve_pages (visible via /proc/zoneinfo "protection" and used > by page allocation fallback and dirty-limit accounting). After this > change only a write does that. Documentation describes the meaning of > the ratio and the derived protection pages, but does not document any > read side-effect. > > Worst case for odd userspace that treated a read as a refresh of those > derived values: lowmem_reserve[] and totalreserve_pages remain at their > last written/setup values until the next write of this sysctl, or until > another existing updater runs (e.g. adjust_managed_page_count() on > managed-page changes, or init/watermark setup paths). Until then, > allocation fallback into lower zones and per-node dirtyable memory > (node_dirtyable_memory() subtracts pgdat->totalreserve_pages) may not > reflect a refresh that such userspace expected from the read alone. > Normal readers that only consume the ratio array are unaffected. > > Signed-off-by: Jianlin Shi > --- > mm/page_alloc.c | 17 +++++++++++------ > 1 file changed, 11 insertions(+), 6 deletions(-) > > diff --git a/mm/page_alloc.c b/mm/page_alloc.c > index 0387d2afd..ce33579ca 100644 > --- a/mm/page_alloc.c > +++ b/mm/page_alloc.c > @@ -6683,16 +6683,21 @@ static int sysctl_min_slab_ratio_sysctl_handler(const struct ctl_table *table, i > static int lowmem_reserve_ratio_sysctl_handler(const struct ctl_table *table, > int write, void *buffer, size_t *length, loff_t *ppos) > { > - int i; > + int i, rc; > > - proc_dointvec_minmax(table, write, buffer, length, ppos); > + rc = proc_dointvec_minmax(table, write, buffer, length, ppos); > + if (rc) > + return rc; > > - for (i = 0; i < MAX_NR_ZONES; i++) { > - if (sysctl_lowmem_reserve_ratio[i] < 1) > - sysctl_lowmem_reserve_ratio[i] = 0; > + if (write) { > + for (i = 0; i < MAX_NR_ZONES; i++) { > + if (sysctl_lowmem_reserve_ratio[i] < 1) > + sysctl_lowmem_reserve_ratio[i] = 0; > + } Could we also get rid of this adjustment with proc_dointvec_minmax's handling of tbl->extra1 set to SYSCTL_ZERO? > + > + setup_per_zone_lowmem_reserve(); > } > > - setup_per_zone_lowmem_reserve(); > return 0; > } >