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De Francesco" To: Andrew Morton , linux-mm@kvack.org, linux-kernel@vger.kernel.org, Ira Weiny Cc: Ira Weiny Subject: Re: [PATCH] mm/memory: Replace kmap() with kmap_local_page() Date: Mon, 18 Dec 2023 08:43:11 +0100 Message-ID: <4255260.irdbgypaU6@fdefranc-mobl3> Organization: intel In-Reply-To: <657fbdb5db945_126a129483@iweiny-mobl.notmuch> References: <20231215084417.2002370-1-fabio.maria.de.francesco@linux.intel.com> <657fbdb5db945_126a129483@iweiny-mobl.notmuch> MIME-Version: 1.0 Content-Transfer-Encoding: 7Bit Content-Type: text/plain; charset="us-ascii" X-Rspamd-Queue-Id: 17E8C140013 X-Rspam-User: X-Rspamd-Server: rspam02 X-Stat-Signature: qki7dsothzmm5tqts3pq4bbhramasizh X-HE-Tag: 1702885398-489497 X-HE-Meta: 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 xuEKXMcZ 1WDEYOUTWlTrtWn8ZpfIL8WaU6e3hq66r3XNUonJZd3U6/7bo3qyZO6r3M5PkWe4+pe77PpKCnNPbdbFxvVrP1F6cnXwaIT8FR+IpDfYt8lMicgTKEGk4eZoIAKVAmnnC/nLPDS+ImFMlX3erI8lP5f7xbjg10QIGGxNtbq/G0Bauj8CUj6hSh4ff847f/HykES0iOVw2H/NW+ezVn4VLF1HUNMK354w6TLd0dvYUZQXHnIm8AR+hKCtRAWbAwHSd68MVF7AZoRgh/Tfy2/VRY01hIenn9qDCx9gKObmO063Yti3UfAfqFDLLwQJGMlWTZ5pcv8kUS3ub4g0LVh9AvQb4UUvObfzFN96rE5kJK4E525q2vDPxhz9I2A== 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 Monday, 18 December 2023 04:34:13 CET Ira Weiny wrote: > Fabio M. De Francesco wrote: > > [snip] > > > Cc: Ira Weiny > > Signed-off-by: Fabio M. De Francesco > > --- > > > > mm/memory.c | 5 ++--- > > 1 file changed, 2 insertions(+), 3 deletions(-) > > > > diff --git a/mm/memory.c b/mm/memory.c > > index 7d9f6b685032..88377a107fbe 100644 > > --- a/mm/memory.c > > +++ b/mm/memory.c > > @@ -5852,7 +5852,7 @@ static int __access_remote_vm(struct mm_struct *mm, > > unsigned long addr,> > > if (bytes > PAGE_SIZE-offset) > > > > bytes = PAGE_SIZE-offset; > > > > - maddr = kmap(page); > > + maddr = kmap_local_page(page); > > > > if (write) { > > > > copy_to_user_page(vma, page, addr, > > > > maddr + offset, buf, bytes); > > > > @@ -5861,8 +5861,7 @@ static int __access_remote_vm(struct mm_struct *mm, > > unsigned long addr,> > > copy_from_user_page(vma, page, addr, > > > > buf, maddr + offset, bytes); > > > > } > > > > - kunmap(page); > > - put_page(page); > > + unmap_and_put_page(page, maddr); > > Does this really have the same functionality? > > Ira Do you have any specific reasons to say that? The unmap_and_put_page() helper was created by Al Viro (it initially was put_and_unmap_page() and I sent a patch to rename it to the current name). He noticed that we have lots of kunmap_local() followed by put_page(). The current implementation has then been changed (Matthew did it, if I remember correctly). My understanding of the current implementation is that unmap_and_put_page() calls folio_release_kmap(), taking as arguments the folio which the page belongs to and the kernel virtual address returned by kmap_local_page(). folio_release_kmap() calls kunmap_local() and then folio_put(). The last is called on the folio obtained by the unmap_and_put_page() wrapper and, if I'm not wrong, it releases refcounts on folios like put_page() does on pages. Am I missing something? For further reference, please take a look at the following path from Al Viro that is modelled after my conversions in fs/sysv: https://lore.kernel.org/all/ 20231213000849.2748576-4-viro@zeniv.linux.org.uk/ Thanks, Fabio