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[2003:cb:c706:6b00:bf49:f14b:380d:f871]) by smtp.gmail.com with ESMTPSA id t25-20020a7bc3d9000000b003fc01495383sm6592524wmj.6.2023.07.28.03.12.58 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Fri, 28 Jul 2023 03:12:58 -0700 (PDT) Message-ID: <13b14aa6-302e-63cc-2a99-f5c22b9931fc@redhat.com> Date: Fri, 28 Jul 2023 12:12:57 +0200 MIME-Version: 1.0 User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:102.0) Gecko/20100101 Thunderbird/102.13.0 Content-Language: en-US From: David Hildenbrand To: John Hubbard , linux-kernel@vger.kernel.org Cc: linux-mm@kvack.org, linux-fsdevel@vger.kernel.org, Andrew Morton , Linus Torvalds , liubo , Peter Xu , Matthew Wilcox , Hugh Dickins , Jason Gunthorpe , stable@vger.kernel.org References: <20230727212845.135673-1-david@redhat.com> <20230727212845.135673-3-david@redhat.com> <55c92738-e402-4657-3d46-162ad2c09d68@nvidia.com> <9de80e22-e89f-2760-34f4-61be5f8fd39c@redhat.com> Organization: Red Hat Subject: Re: [PATCH v1 2/4] mm/gup: Make follow_page() succeed again on PROT_NONE PTEs/PMDs In-Reply-To: <9de80e22-e89f-2760-34f4-61be5f8fd39c@redhat.com> Content-Type: text/plain; charset=UTF-8; format=flowed Content-Transfer-Encoding: 7bit Precedence: bulk List-ID: X-Mailing-List: linux-fsdevel@vger.kernel.org On 28.07.23 11:08, David Hildenbrand wrote: > On 28.07.23 04:30, John Hubbard wrote: >> On 7/27/23 14:28, David Hildenbrand wrote: >>> We accidentally enforced PROT_NONE PTE/PMD permission checks for >>> follow_page() like we do for get_user_pages() and friends. That was >>> undesired, because follow_page() is usually only used to lookup a currently >>> mapped page, not to actually access it. Further, follow_page() does not >>> actually trigger fault handling, but instead simply fails. >> >> I see that follow_page() is also completely undocumented. And that >> reduces us to deducing how it should be used...these things that >> change follow_page()'s behavior maybe should have a go at documenting >> it too, perhaps. > > I can certainly be motivated to do that. :) > >> >>> >>> Let's restore that behavior by conditionally setting FOLL_FORCE if >>> FOLL_WRITE is not set. This way, for example KSM and migration code will >>> no longer fail on PROT_NONE mapped PTEs/PMDS. >>> >>> Handling this internally doesn't require us to add any new FOLL_FORCE >>> usage outside of GUP code. >>> >>> While at it, refuse to accept FOLL_FORCE: we don't even perform VMA >>> permission checks like in check_vma_flags(), so especially >>> FOLL_FORCE|FOLL_WRITE would be dodgy. >>> >>> This issue was identified by code inspection. We'll add some >>> documentation regarding FOLL_FORCE next. >>> >>> Reported-by: Peter Xu >>> Fixes: 474098edac26 ("mm/gup: replace FOLL_NUMA by gup_can_follow_protnone()") >>> Cc: >>> Signed-off-by: David Hildenbrand >>> --- >>> mm/gup.c | 10 +++++++++- >>> 1 file changed, 9 insertions(+), 1 deletion(-) >>> >>> diff --git a/mm/gup.c b/mm/gup.c >>> index 2493ffa10f4b..da9a5cc096ac 100644 >>> --- a/mm/gup.c >>> +++ b/mm/gup.c >>> @@ -841,9 +841,17 @@ struct page *follow_page(struct vm_area_struct *vma, unsigned long address, >>> if (vma_is_secretmem(vma)) >>> return NULL; >>> >>> - if (WARN_ON_ONCE(foll_flags & FOLL_PIN)) >>> + if (WARN_ON_ONCE(foll_flags & (FOLL_PIN | FOLL_FORCE))) >>> return NULL; >> >> This is not a super happy situation: follow_page() is now prohibited >> (see above: we should document that interface) from passing in >> FOLL_FORCE... > > I guess you saw my patch #4. > > If you take a look at the existing callers (that are fortunately very > limited), you'll see that nobody cares. > > Most of the FOLL flags don't make any sense for follow_page(), and > limiting further (ab)use is at least to me very appealing. > >> >>> >>> + /* >>> + * Traditionally, follow_page() succeeded on PROT_NONE-mapped pages >>> + * but failed follow_page(FOLL_WRITE) on R/O-mapped pages. Let's >>> + * keep these semantics by setting FOLL_FORCE if FOLL_WRITE is not set. >>> + */ >>> + if (!(foll_flags & FOLL_WRITE)) >>> + foll_flags |= FOLL_FORCE; >>> + >> >> ...but then we set it anyway, for special cases. It's awkward because >> FOLL_FORCE is not an "internal to gup" flag (yet?). >> >> I don't yet have suggestions, other than: >> >> 1) Yes, the FOLL_NUMA made things bad. >> >> 2) And they are still very confusing, especially the new use of >> FOLL_FORCE. >> >> ...I'll try to let this soak in and maybe recommend something >> in a more productive way. :) > > What I can offer that might be very appealing is the following: > > Get rid of the flags parameter for follow_page() *completely*. Yes, then > we can even rename FOLL_ to something reasonable in the context where it > is nowadays used ;) > > > Internally, we'll then set > > FOLL_GET | FOLL_DUMP | FOLL_FORCE > > and document exactly what this functions does. Any user that needs > something different should just look into using get_user_pages() instead. > > I can prototype that on top of this work easily. The end result looks something like: /** * follow_page - look up and reference a page descriptor from a user-virtual * address * @vma: vm_area_struct mapping @address * @address: virtual address to look up * * follow_page() will look up the page mapped at the given address and * take a reference on the page. The returned page has to be released using * put_page(). * * follow_page() will not return special (like zero) pages and does not check * PTE protection: the returned page might be mapped PROT_NONE, R/O or R/W. * Consequently, follow_page() will not trigger NUMA hinting faults. * * follow_page() does not trigger page faults. If no page is mapped, or * a special (like zero) page is mapped, it returns %NULL or an error pointer. * * Note: new users with different requirements are probably better off using * one of the get_user_pages() variants or one of the walk_page_range() * variants. * * Return: the mapped (struct page *), %NULL if no mapping exists, or * an error pointer if there is a mapping to something not represented * by a page descriptor (see also vm_normal_page()) or the zero page. */ struct page *follow_page(struct vm_area_struct *vma, unsigned long address) { struct follow_page_context ctx = { NULL }; unsigned long gup_flags; struct page *page; if (vma_is_secretmem(vma)) return NULL; /* * FOLL_GET: We always want a reference on the returned page. * FOL_DUMP: Ignore special (like zero) pages. * FOLL_FORCE: Succeeded on PROT_NONE-mapped pages. */ gup_flags = FOLL_GET | FOLL_DUMP | FOLL_FORCE; page = follow_page_mask(vma, address, gup_flags, &ctx); if (ctx.pgmap) put_dev_pagemap(ctx.pgmap); return page; } -- Cheers, David / dhildenb