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[86.164.143.89]) by smtp.gmail.com with ESMTPSA id m38-20020a05600c3b2600b00412b6fbb9b5sm13627109wms.8.2024.03.10.15.16.17 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Sun, 10 Mar 2024 15:16:18 -0700 (PDT) Date: Sun, 10 Mar 2024 22:14:04 +0000 From: Lorenzo Stoakes To: Richard Weinberger Cc: linux-mm@kvack.org, linux-fsdevel@vger.kernel.org, linux-kernel@vger.kernel.org, linux-doc@vger.kernel.org, upstream+pagemap@sigma-star.at, adobriyan@gmail.com, wangkefeng.wang@huawei.com, ryan.roberts@arm.com, hughd@google.com, peterx@redhat.com, david@redhat.com, avagin@google.com, vbabka@suse.cz, akpm@linux-foundation.org, usama.anjum@collabora.com, corbet@lwn.net Subject: Re: [PATCH 2/2] [RFC] pagemap.rst: Document write bit Message-ID: References: <20240306232339.29659-1-richard@nod.at> <20240306232339.29659-2-richard@nod.at> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20240306232339.29659-2-richard@nod.at> X-Rspamd-Queue-Id: 1F10B100008 X-Rspam-User: X-Rspamd-Server: rspam05 X-Stat-Signature: kbkf686x6ppe14kox646uibu1fq7o5x1 X-HE-Tag: 1710108980-668191 X-HE-Meta: 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 57kKvyH8 ZiDGM+F72WvRUcICPIELOOqTmHWuyUkUlnTlOvCk4l8htRha0kxf1K1z2tFNHh9sYHyU4PQqrNgVUJGKs1WMK0kIjWqMpxziySSD32AmVx+DlO3VyQCFQQ2I9T/4cnwdFu3BlujcRd1GuIpcrHVEk5PLoNw66CeWJsp0b 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: List-Subscribe: List-Unsubscribe: On Thu, Mar 07, 2024 at 12:23:39AM +0100, Richard Weinberger wrote: > Bit 58 denotes that a PTE is writable. > The main use case is detecting CoW mappings. > > Signed-off-by: Richard Weinberger > --- > Documentation/admin-guide/mm/pagemap.rst | 8 +++++++- > 1 file changed, 7 insertions(+), 1 deletion(-) > > diff --git a/Documentation/admin-guide/mm/pagemap.rst b/Documentation/admin-guide/mm/pagemap.rst > index f5f065c67615..81ffe3601b96 100644 > --- a/Documentation/admin-guide/mm/pagemap.rst > +++ b/Documentation/admin-guide/mm/pagemap.rst > @@ -21,7 +21,8 @@ There are four components to pagemap: > * Bit 56 page exclusively mapped (since 4.2) > * Bit 57 pte is uffd-wp write-protected (since 5.13) (see > Documentation/admin-guide/mm/userfaultfd.rst) > - * Bits 58-60 zero > + * Bit 58 pte is writable (since 6.10) I really think we need to be careful about talking about 'writable' again because people are easily confused about the difference between a writable _mapping_ and a writable _page table entry_. Of course you mention PTE here, but I think it might be better to say something like: * Bit 58 raw pte r/w flag (since 6.10) > + * Bits 59-60 zero > * Bit 61 page is file-page or shared-anon (since 3.5) > * Bit 62 page swapped > * Bit 63 page present > @@ -37,6 +38,11 @@ There are four components to pagemap: > precisely which pages are mapped (or in swap) and comparing mapped > pages between processes. > > + Bit 58 is useful to detect CoW mappings; however, it does not indicate > + whether the page mapping is writable or not. If an anonymous mapping is > + writable but the write bit is not set, it means that the next write access > + will cause a page fault, and copy-on-write will happen. > + David has addressed the copy vs. anon exclusive remap issue, but I also feel this needs some balking out. I would simply rephrase this in terms of whether a write fault occurs or not e.g.: Bit 58 indicates whether the PTE has the write flag set. If this flag is unset, then write accesses for this mapping will cause a fault for this page. If the mapping is private (whether anonymous or file-backed), this can result in a Copy-on-Write (though if anonymous-excusive the flag will simply be set). If file-backed, this being cleared may simply indicate that this file page is clean. > Efficient users of this interface will use ``/proc/pid/maps`` to > determine which areas of memory are actually mapped and llseek to > skip over unmapped regions. > -- > 2.35.3 >