From mboxrd@z Thu Jan 1 00:00:00 1970 From: David Hildenbrand Subject: Re: [PATCH RFC] mm/madvise: introduce MADV_POPULATE to prefault/prealloc memory Date: Thu, 18 Feb 2021 12:38:27 +0100 Message-ID: <740cdd51-137b-2b08-8b7f-9757d8d847cb@redhat.com> References: <20210217154844.12392-1-david@redhat.com> <3763a505-02d6-5efe-a9f5-40381acfbdfd@redhat.com> Mime-Version: 1.0 Content-Transfer-Encoding: 8bit Return-path: DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=redhat.com; s=mimecast20190719; t=1613648326; 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: content-transfer-encoding:content-transfer-encoding: in-reply-to:in-reply-to:references:references; bh=3A4zaSGP/jDU0jQya7nJb+PtO132rNXYV3X5vurWHRM=; b=FsnxXWc2e03gNM9vf6/ZY/Yo7E9exlCDot+KyryDAdQ2JH3jarJj+EpjyHE1+DvUCQVup8 ruVnGUhFmzEx1y/JrlRJ9ko6K/iUOsnq5hh/NMrucNOPrVVyjvSihc7lQZVxqot01/fVmd 8hLWbHxsl5xzjkam9TZ7EQaVOA5WyIk= In-Reply-To: Content-Language: en-US List-ID: Content-Type: text/plain; charset="us-ascii"; format="flowed" To: Michal Hocko Cc: linux-kernel@vger.kernel.org, linux-mm@kvack.org, Andrew Morton , Arnd Bergmann , Oscar Salvador , Matthew Wilcox , Andrea Arcangeli , Minchan Kim , Jann Horn , Jason Gunthorpe , Dave Hansen , Hugh Dickins , Rik van Riel , "Michael S . Tsirkin" , "Kirill A . Shutemov" , Vlastimil Babka , Richard Henderson , Ivan Kokshaysky , Matt Turner , Thomas Bogendoerfer , James E.J. Bottomle >>>> If we hit >>>> hardware errors on pages, ignore them - nothing we really can or >>>> should do. >>>> 3. On errors during MADV_POPULATED, some memory might have been >>>> populated. Callers have to clean up if they care. >>> >>> How does caller find out? madvise reports 0 on success so how do you >>> find out how much has been populated? >> >> If there is an error, something might have been populated. In my QEMU >> implementation, I simply discard the range again, good enough. I don't think >> we need to really indicate "error and populated" or "error and not >> populated". > > Agreed. The wording just suggests that the syscall actually provides any > means for an effective way to handle those errors. Maybe you should just > stick with the first sentence and drop the second. Makes sense. "On errors during MADV_POPULATE, some memory might have been populated." > >>>> 4. Concurrent changes to the virtual memory layour are tolerated - we >>>> process each and every PFN only once, though. >>> >>> I do not understand this. madvise is about virtual address space not a >>> physical address space. >> >> What I wanted to express: if we detect a change in the mapping we don't >> restart at the beginning, we always make forward progress. We process each >> virtual address once (on a per-page basis, thus I accidentally used "PFN"). > > This is an implicit assumption. Your range can have the same page mapped > several times in the given address range and all you care about is that > you fault those which are not present during the virtual address space > walk. Your syscall can return and large part of the address space might > be unpopulated because memory reclaim just dropped those pages and that > would be fine. This shouldn't really imply memory presence - mlock does > that. "Concurrent changes to the virtual memory layout are tolerated. The range is processed exactly once." > >>>> 5. If MADV_POPULATE succeeds, all memory in the range can be accessed >>>> without SIGBUS. (of course, not if user space changed mappings in the >>>> meantime or KSM kicked in on anonymous memory). >>> >>> I do not see how KSM would change anything here and maybe it is not >>> really important to mention it. KSM should be really transparent from >>> the users space POV. Parallel and destructive virtual address space >>> operations are also expected to change the outcome and there is nothing >>> kernel do about at and provide any meaningful guarantees. I guess we >>> want to assume a reasonable userspace behavior here. >> >> It's just a note that we cannot protect from someone interfering >> (discard/ksm/whatever). I'm making that clearer in the cover letter. > > Again that is implicit expectation. madvise will not work for anybody > shooting an own foot. Okay, I'll drop that part, thanks! -- Thanks, David / dhildenb