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Tsirkin" , Jason Wang , Vlastimil Babka , =?utf-8?b?SsOpcsO0bWUgR2xpc3Nl?= , Jan Kara , Dave Chinner , Souptick Joarder , Jonathan Corbet , , , , , , LKML , , John Hubbard Subject: [PATCH v2 1/2] docs: mm/gup: pin_user_pages.rst: add a "case 5" Date: Sun, 31 May 2020 22:26:32 -0700 Message-ID: <20200601052633.853874-2-jhubbard@nvidia.com> X-Mailer: git-send-email 2.26.2 In-Reply-To: <20200601052633.853874-1-jhubbard@nvidia.com> References: <20200601052633.853874-1-jhubbard@nvidia.com> MIME-Version: 1.0 X-NVConfidentiality: public DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=nvidia.com; s=n1; t=1590989113; bh=t7FWcHHh76dL2ntq5uEZcJXrRdIPUbkPCP1hpf5ZyJs=; h=X-PGP-Universal:From:To:CC:Subject:Date:Message-ID:X-Mailer: In-Reply-To:References:MIME-Version:X-NVConfidentiality: Content-Type:Content-Transfer-Encoding; b=qK1j9GFqbfQmVpQy1H0RYKNlXRO1BkoJ29m7kmpMnNPQuUtvQfBwg2vMMsSvFveYZ B4RJF6yYzDWJPbA+0xaMNCl5pIiwE9zxqS3DmytXMMVogrqpR6QR5ifPBK/4OdqkAx 5sQJ9sxAAQ3GrAOxexuMi6vvC13lWsy/ISJCnIhPlJoIMi/2csU1MFDnTggQ4T94oK Tod2zq/E8lTiWlfYGrPfl+Wiu7+lAkY4xGFHT6UWkLNH8s/BwSkflVnW/N7ALhfKyH hwASUzab4eDY3/262l16OFARt1OWnabnYoH3Ab53hs03ClSVHIf+sK0Fj9TGdPcb8l GptD7xNG4tqcw== X-Rspamd-Queue-Id: B8B1B180AD801 X-Spamd-Result: default: False [0.00 / 100.00] X-Rspamd-Server: rspam04 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: There are four cases listed in pin_user_pages.rst. These are intended to help developers figure out whether to use get_user_pages*(), or pin_user_pages*(). However, the four cases do not cover all the situations. For example, drivers/vhost/vhost.c has a "pin, write to page, set page dirty, unpin" case. Add a fifth case, to help explain that there is a general pattern that requires pin_user_pages*() API calls. Cc: Vlastimil Babka Cc: Jan Kara Cc: Jérôme Glisse Cc: Dave Chinner Cc: Jonathan Corbet Cc: linux-doc@vger.kernel.org Cc: linux-fsdevel@vger.kernel.org Signed-off-by: John Hubbard Reviewed-by: Jan Kara --- Documentation/core-api/pin_user_pages.rst | 18 ++++++++++++++++++ 1 file changed, 18 insertions(+) diff --git a/Documentation/core-api/pin_user_pages.rst b/Documentation/core-api/pin_user_pages.rst index 4675b04e8829..6068266dd303 100644 --- a/Documentation/core-api/pin_user_pages.rst +++ b/Documentation/core-api/pin_user_pages.rst @@ -171,6 +171,24 @@ If only struct page data (as opposed to the actual memory contents that a page is tracking) is affected, then normal GUP calls are sufficient, and neither flag needs to be set. +CASE 5: Pinning in order to write to the data within the page +------------------------------------------------------------- +Even though neither DMA nor Direct IO is involved, just a simple case of "pin, +write to a page's data, unpin" can cause a problem. Case 5 may be considered a +superset of Case 1, plus Case 2, plus anything that invokes that pattern. In +other words, if the code is neither Case 1 nor Case 2, it may still require +FOLL_PIN, for patterns like this: + +Correct (uses FOLL_PIN calls): + pin_user_pages() + write to the data within the pages + unpin_user_pages() + +INCORRECT (uses FOLL_GET calls): + get_user_pages() + write to the data within the pages + put_page() + page_maybe_dma_pinned(): the whole point of pinning ===================================================