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[2003:cb:c708:1700:e40d:574c:c991:5f78]) by smtp.gmail.com with ESMTPSA id j28-20020a5d6e5c000000b0022cd27bc8c1sm23189459wrz.9.2022.10.21.07.10.41 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Fri, 21 Oct 2022 07:10:41 -0700 (PDT) Message-ID: Date: Fri, 21 Oct 2022 16:10:41 +0200 MIME-Version: 1.0 User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:102.0) Gecko/20100101 Thunderbird/102.3.1 To: Peter Xu Cc: Matthew Wilcox , linux-mm@kvack.org, Hugh Dickins References: From: David Hildenbrand Organization: Red Hat Subject: Re: Avoiding allocation of unused shmem page In-Reply-To: X-Mimecast-Spam-Score: 0 X-Mimecast-Originator: redhat.com Content-Language: en-US Content-Type: text/plain; charset=UTF-8; format=flowed Content-Transfer-Encoding: 7bit ARC-Seal: i=1; s=arc-20220608; d=hostedemail.com; t=1666361446; a=rsa-sha256; cv=none; b=bO2ET+dR8C0H+qWgr8fyESVhMp8S8lkPNSmtZN63wcsYIJsIftQ/MlxaDPmfBFBR97uL7Y ED2GL6cAAzWq3wedazq28p/lhon7JMDetZ4E/jHhzjOS8gUOATWWdquRPBCuBhCI4sIV3e jLp7dR+fWhbaCdRU93lTd5qQp6JUeT0= ARC-Authentication-Results: i=1; imf25.hostedemail.com; dkim=pass header.d=redhat.com header.s=mimecast20190719 header.b=Z96UrRW+; dmarc=pass (policy=none) header.from=redhat.com; spf=pass (imf25.hostedemail.com: domain of david@redhat.com designates 170.10.129.124 as permitted sender) smtp.mailfrom=david@redhat.com ARC-Message-Signature: i=1; a=rsa-sha256; c=relaxed/relaxed; d=hostedemail.com; s=arc-20220608; t=1666361446; h=from:from:sender: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:dkim-signature; bh=gnXBwe+gjGfxWXsecpDPrfeTYART/drCxusIY4wrBYA=; b=qfGH1OIMil5or5kDGhGqVatkhYkKr4jLuO9nDaotGfsLNW9V5AodbhtSVR7NekhHzHg8c3 njUJ32FX24PEbLJUa46X+5PysMJ7MdsOYo7Xbjfwf8bJbKUlJU+19l5HTGGfI5+XfarVs3 RM79QdyFrpiMY3RkAOG0T5WmUtaoqJM= X-Rspamd-Queue-Id: 27F62A003F Authentication-Results: imf25.hostedemail.com; dkim=pass header.d=redhat.com header.s=mimecast20190719 header.b=Z96UrRW+; dmarc=pass (policy=none) header.from=redhat.com; spf=pass (imf25.hostedemail.com: domain of david@redhat.com designates 170.10.129.124 as permitted sender) smtp.mailfrom=david@redhat.com X-Rspam-User: X-Rspamd-Server: rspam08 X-Stat-Signature: jfpdhjkr3ajfjrrgekawd8nmi4ajt39t X-HE-Tag: 1666361445-79238 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 21.10.22 16:01, Peter Xu wrote: > On Fri, Oct 21, 2022 at 09:23:00AM +0200, David Hildenbrand wrote: >> On 20.10.22 23:10, Peter Xu wrote: >>> On Thu, Oct 20, 2022 at 09:14:09PM +0100, Matthew Wilcox wrote: >>>> In yesterday's call, David brought up the case where we fallocate a file >>>> in shmem, call mmap(MAP_PRIVATE) and then store to a page which is over >>>> a hole. That currently causes shmem to allocate a page, zero-fill it, >>>> then COW it, resulting in two pages being allocated when only the >>>> COW page really needs to be allocated. >>>> >>>> The path we currently take through the MM when we take the page fault >>>> looks like this (correct me if I'm wrong ...): >>>> >>>> handle_mm_fault() >>>> __handle_mm_fault() >>>> handle_pte_fault() >>>> do_fault() >>>> do_cow_fault() >>>> __do_fault() >>>> vm_ops->fault() >>>> >>>> ... which is where we come into shmem_fault(). Apart from the >>>> horrendous hole-punch handling case, shmem_fault() is quite simple: >>>> >>>> err = shmem_get_folio_gfp(inode, vmf->pgoff, &folio, SGP_CACHE, >>>> gfp, vma, vmf, &ret); >>>> if (err) >>>> return vmf_error(err); >>>> vmf->page = folio_file_page(folio, vmf->pgoff); >>>> return ret; >>>> >>>> What we could do here is detect this case. Something like: >>>> >>>> enum sgp_type sgp = SGP_CACHE; >>>> >>>> if ((vmf->flags & FAULT_FLAG_WRITE) && !(vma->vm_flags & VM_SHARED)) >>>> sgp = SGP_READ; >>> >>> Yes this will start to save the space, but just to mention this may start >>> to break anything that will still depend on the pagecache to work. E.g., >>> it'll change behavior if the vma is registered with uffd missing mode; >>> we'll start to lose MISSING events for these private mappings. Not sure >>> whether there're other side effects. >> >> I don't follow, can you elaborate? >> >> hugetlb doesn't perform this kind of unnecessary allocation and should be fine in regards to uffd. Why should it matter here and how exactly would a problematic sequence look like? > > Hugetlb is special because hugetlb detects pte first and relies on pte at > least for uffd. shmem is not. > > Feel free to also reference the recent fix which relies on the stable > hugetlb pte with commit 2ea7ff1e39cbe375. Sorry to be dense here, but I don't follow how that relates. Assume we have a MAP_PRIVATE shmem mapping and someone registers uffd missing events on that mapping. Assume we get a page fault on a hole. We detect no page is mapped and check if the page cache has a page mapped -- which is also not the case, because there is a hole. So we notify uffd. Uffd will place a page. It should *not* touch the page cache and only insert that page into the page table -- otherwise we'd be violating MAP_PRIVATE semantics. What am I missing? [...] >> >> There is an easy way to trigger this from QEMU, and we've had >> customers running into this: > > Can the customer simply set shared=on? > Of course they can. It rather comes with a surprise for them, because -- for now -- we're not even warning that this most probably doesn't make too much sense. -- Thanks, David / dhildenb