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c=relaxed/relaxed; d=1e100.net; s=20161025; h=x-gm-message-state:mime-version:references:in-reply-to:from:date :message-id:subject:to:cc:content-transfer-encoding; bh=yxVOjFjkk/aWg0934gm436IS5GNuiS+Phez+14sm9BM=; b=DhZjWX7lGb/0kR1tc9DpM08NRQRsd9NFbaI7To/Jl8cmaqfoCOfwGcPIAKSIZqnCKt Jk2XCwKK69b2FeLg1moa/A0AuX9Wa3hWQY0AFzcnDbqF7IdoADV2uE02QYq4tNxXO0Hx rEV+Zv8FSzq38ajtLdRys3ZYiC4jqaINmsYSdGX6jJsPCxQAlxn/Wol0P075b5wyVOrT oj4ALqyHk0vxIsaFtT0uESywBoyWMXzKBzVwtoOilU16Jf7C1yTcmpkOZQzct/KqMtCP KLxV6OVVZrpkys+v0GmibJseM7JQoyLHsndMBc1UirsU+L+bXD11xtF/BkMV3o06aNFX s0OA== X-Gm-Message-State: AOAM531GXoySyRU3UHfpNWxXmp103UkIZ4Up+T0kpS75gFgddZtQsyHt hppQJ3oeM1omJAXcF3kQuNo2Qir0L4YUo94FgHZzxA== X-Google-Smtp-Source: ABdhPJz/ws+tQdTlD2sro4O3KJY0G2O9Z2dFSKkGdI9/xqHwUEDbMwkiwcMYMZHYYK/aGD6hzY3rio10qCe6eKIGuDc= X-Received: by 2002:a1c:980a:: with SMTP id a10mr216873wme.103.1603993037592; Thu, 29 Oct 2020 10:37:17 -0700 (PDT) MIME-Version: 1.0 References: <20201026173358.14704-1-vbabka@suse.cz> <20201026173358.14704-4-vbabka@suse.cz> <93ab79df-cf8c-294b-3ed1-8a563e4a452b@redhat.com> <1fc7ec3a-367c-eb9f-1cb4-b9e015fea87c@suse.cz> <81faf3d6-9536-ff00-447d-e964a010492d@suse.cz> In-Reply-To: <81faf3d6-9536-ff00-447d-e964a010492d@suse.cz> From: Alexander Potapenko Date: Thu, 29 Oct 2020 18:37:04 +0100 Message-ID: Subject: Re: [PATCH 3/3] mm, page_alloc: reduce static keys in prep_new_page() To: Vlastimil Babka Cc: David Hildenbrand , Andrew Morton , Linux Memory Management List , LKML , Kees Cook , Michal Hocko , Mateusz Nosek , Laura Abbott Content-Type: text/plain; charset="UTF-8" Content-Transfer-Encoding: quoted-printable 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: On Tue, Oct 27, 2020 at 2:32 PM Vlastimil Babka wrote: > > On 10/27/20 12:05 PM, Vlastimil Babka wrote: > > On 10/27/20 10:10 AM, David Hildenbrand wrote: > >> On 26.10.20 18:33, Vlastimil Babka wrote: > >>> prep_new_page() will always zero a new page (regardless of __GFP_ZERO= ) when > >>> init_on_alloc is enabled, but will also always skip zeroing if the pa= ge was > >>> already zeroed on free by init_on_free or page poisoning. > >>> > >>> The latter check implemented by free_pages_prezeroed() can involve tw= o > >>> different static keys. As prep_new_page() is really a hot path, let's= introduce > >>> a single static key free_pages_not_prezeroed for this purpose and ini= tialize it > >>> in init_mem_debugging(). > >> > >> Is this actually observable in practice? This smells like > >> micro-optimization to me. > >> > >> Also, I thought the whole reason for static keys is to have basically = no > >> overhead at runtime, so I wonder if replacing two static key checks by= a > >> single one actually makes *some* difference. > > > > You're right, the difference seems to be just a single NOP. The static = key > > infrastructure seems to be working really well. > > (At least the asm inspection made me realize that kernel_poison_pages()= is > > called unconditionally and the static key is checked inside, not inline= so I'll > > be amending patch 2...) > > > > Initially I thought I would be reducing 3 keys to 1 in this patch, but = I got the > > code wrong. So unless others think it's a readability improvements, we = can drop > > this patch. I agree with David that replacing two static keys with one is probably a micro-optimization. Also, if someone is enabling both init_on_alloc and init_on_free, they are already paying so much that no one is going to notice an extra static key. > > Or we can also reconsider this whole optimization. If the point is to b= e > > paranoid and enable both init_on_free and init_on_alloc, should we trus= t that > > nobody wrote something after the clearing on free via use-after-free? := ) Kees/Alex? I think we must trust the kernel to not overwrite zeroed pages. After all, this could theoretically happen at any time, not only while the memory chunk is freed. > More thoughts... > > PAGE_POISONING_NO_SANITY skips the check on "unpoisoning" whether poison = was > corrupted > PAGE_POISONING_ZERO uses zero instead of 0xAA as poison pattern > > the point of enabling both of these seems to be moot now that init_on_fre= e > exists, as that zeroes pages that are being freed, without checking on al= loc > that they are still zeroed. > > What if only one is enabled? > - PAGE_POISONING_NO_SANITY without PAGE_POISONING_ZERO - we poison with t= he 0xAA > pattern but nobody checks it, so does it give us anything over init_on_fr= ee > writing zeroes? I don't think so? > > - PAGE_POISONING_ZERO without PAGE_POISONING_NO_SANITY - we use zeroes (l= ike > init_on_free) but also check that it wasn't corrupted. We save some time = on > writing zeroes again on alloc, but the check is still expensive. And writ= ing > 0xAA would possibly detect more corruptions than writing zero (a stray wr= ite of > NULL is more likely to happen than of 0xAA?). > > So my conclusion: > - We can remove PAGE_POISONING_NO_SANITY because it only makes sense with > PAGE_POISONING_ZERO, and we can use init_on_free instead Agreed. > - We can also probably remove PAGE_POISONING_ZERO, because if we want to = do the > unpoisoning sanity check, then we also most likely want the 0xAA pattern = and not > zero. Agreed. It might also make sense to somehow merge page poisoning and init_on_free together and have one config dimension instead of two (providing something similar to the INIT_STACK_NONE/INIT_STACK_ALL_ZERO/INIT_STACK_ALL_PATTERN configs) > Thoughts? > --=20 Alexander Potapenko Software Engineer Google Germany GmbH Erika-Mann-Stra=C3=9Fe, 33 80636 M=C3=BCnchen Gesch=C3=A4ftsf=C3=BChrer: Paul Manicle, Halimah DeLaine Prado Registergericht und -nummer: Hamburg, HRB 86891 Sitz der Gesellschaft: Hamburg