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bh=n2MkOxcvB8unEejbMWkMEf534o0h2+e07KJxot0+dsE=; b=REQe703Uk6gd81YHTqgZekG1+bSzV+Z8Mp+d0cNgwfbaKiIMfpt+RCa2esZeUnKT53R/wY 7/+0VwbN2fwzoWTtVzyCtY0M1M9DEVZcM8Vt5aJBsG85WEE5TyNVTSJMEWZDu95sXPbvOJ mw97JaHaCRyNICL1UWH5DfzULuCovPg= Received: from mimecast-mx02.redhat.com (mimecast-mx02.redhat.com [66.187.233.88]) by relay.mimecast.com with ESMTP with STARTTLS (version=TLSv1.2, cipher=TLS_ECDHE_RSA_WITH_AES_256_GCM_SHA384) id us-mta-253-uC0SAgkeM7aQ7FeHN1dj7w-1; Wed, 08 Mar 2023 11:53:08 -0500 X-MC-Unique: uC0SAgkeM7aQ7FeHN1dj7w-1 Received: from smtp.corp.redhat.com (int-mx06.intmail.prod.int.rdu2.redhat.com [10.11.54.6]) (using TLSv1.2 with cipher AECDH-AES256-SHA (256/256 bits)) (No client certificate requested) by mimecast-mx02.redhat.com (Postfix) with ESMTPS id EBC6F886C61; Wed, 8 Mar 2023 16:53:06 +0000 (UTC) Received: from warthog.procyon.org.uk (unknown [10.33.36.18]) by smtp.corp.redhat.com (Postfix) with ESMTP id 8E7602166B26; Wed, 8 Mar 2023 16:53:04 +0000 (UTC) From: David Howells To: Jens Axboe , Al Viro , Christoph Hellwig Cc: David Howells , Matthew Wilcox , Jan Kara , Jeff Layton , David Hildenbrand , Jason Gunthorpe , Logan Gunthorpe , Hillf Danton , Linus Torvalds , linux-fsdevel@vger.kernel.org, linux-block@vger.kernel.org, linux-kernel@vger.kernel.org, linux-mm@kvack.org, Daniel Golle , Guenter Roeck , Christoph Hellwig , John Hubbard , Hugh Dickins Subject: [PATCH v17 03/14] shmem: Implement splice-read Date: Wed, 8 Mar 2023 16:52:40 +0000 Message-Id: <20230308165251.2078898-4-dhowells@redhat.com> In-Reply-To: <20230308165251.2078898-1-dhowells@redhat.com> References: <20230308165251.2078898-1-dhowells@redhat.com> MIME-Version: 1.0 X-Scanned-By: MIMEDefang 3.1 on 10.11.54.6 X-Rspamd-Queue-Id: DED5AC0003 X-Rspamd-Server: rspam09 X-Rspam-User: X-Stat-Signature: wcr4jdpcckgbg9b5ugoynbu59onjyn4r X-HE-Tag: 1678294391-359604 X-HE-Meta: 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 6zu2rD0l 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 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: The new filemap_splice_read() has an implicit expectation via filemap_get_pages() that ->read_folio() exists if ->readahead() doesn't fully populate the pagecache of the file it is reading from[1], potentially leading to a jump to NULL if this doesn't exist. shmem, however, (and by extension, tmpfs, ramfs and rootfs), doesn't have ->read_folio(), Work around this by equipping shmem with its own splice-read implementation, based on filemap_splice_read(), but able to paste in zero_page when there's a page missing. Signed-off-by: David Howells cc: Daniel Golle cc: Guenter Roeck cc: Christoph Hellwig cc: Jens Axboe cc: Al Viro cc: John Hubbard cc: David Hildenbrand cc: Matthew Wilcox cc: Hugh Dickins cc: linux-block@vger.kernel.org cc: linux-fsdevel@vger.kernel.org cc: linux-mm@kvack.org Link: https://lore.kernel.org/r/Y+pdHFFTk1TTEBsO@makrotopia.org/ [1] --- mm/shmem.c | 124 ++++++++++++++++++++++++++++++++++++++++++++++++++++- 1 file changed, 123 insertions(+), 1 deletion(-) diff --git a/mm/shmem.c b/mm/shmem.c index 448f393d8ab2..3cbec1d56112 100644 --- a/mm/shmem.c +++ b/mm/shmem.c @@ -2719,6 +2719,128 @@ static ssize_t shmem_file_read_iter(struct kiocb *iocb, struct iov_iter *to) return retval ? retval : error; } +static bool zero_pipe_buf_try_steal(struct pipe_inode_info *pipe, + struct pipe_buffer *buf) +{ + return false; +} + +static const struct pipe_buf_operations zero_pipe_buf_ops = { + .release = generic_pipe_buf_release, + .try_steal = zero_pipe_buf_try_steal, + .get = generic_pipe_buf_get, +}; + +static size_t splice_zeropage_into_pipe(struct pipe_inode_info *pipe, + loff_t fpos, size_t size) +{ + size_t offset = fpos & ~PAGE_MASK; + + size = min_t(size_t, size, PAGE_SIZE - offset); + + if (!pipe_full(pipe->head, pipe->tail, pipe->max_usage)) { + struct pipe_buffer *buf = pipe_head_buf(pipe); + + *buf = (struct pipe_buffer) { + .ops = &zero_pipe_buf_ops, + .page = ZERO_PAGE(0), + .offset = offset, + .len = size, + }; + get_page(buf->page); + pipe->head++; + } + + return size; +} + +static ssize_t shmem_file_splice_read(struct file *in, loff_t *ppos, + struct pipe_inode_info *pipe, + size_t len, unsigned int flags) +{ + struct inode *inode = file_inode(in); + struct address_space *mapping = inode->i_mapping; + struct folio *folio = NULL; + size_t total_spliced = 0, used, npages, n, part; + loff_t isize; + int error = 0; + + /* Work out how much data we can actually add into the pipe */ + used = pipe_occupancy(pipe->head, pipe->tail); + npages = max_t(ssize_t, pipe->max_usage - used, 0); + len = min_t(size_t, len, npages * PAGE_SIZE); + + do { + if (*ppos >= i_size_read(inode)) + break; + + error = shmem_get_folio(inode, *ppos / PAGE_SIZE, &folio, SGP_READ); + if (error) { + if (error == -EINVAL) + error = 0; + break; + } + if (folio) { + folio_unlock(folio); + + if (folio_test_hwpoison(folio)) { + error = -EIO; + break; + } + } + + /* + * i_size must be checked after we know the pages are Uptodate. + * + * Checking i_size after the check allows us to calculate + * the correct value for "nr", which means the zero-filled + * part of the page is not copied back to userspace (unless + * another truncate extends the file - this is desired though). + */ + isize = i_size_read(inode); + if (unlikely(*ppos >= isize)) + break; + part = min_t(loff_t, isize - *ppos, len); + + if (folio) { + /* + * If users can be writing to this page using arbitrary + * virtual addresses, take care about potential aliasing + * before reading the page on the kernel side. + */ + if (mapping_writably_mapped(mapping)) + flush_dcache_folio(folio); + folio_mark_accessed(folio); + /* + * Ok, we have the page, and it's up-to-date, so we can + * now splice it into the pipe. + */ + n = splice_folio_into_pipe(pipe, folio, *ppos, part); + folio_put(folio); + folio = NULL; + } else { + n = splice_zeropage_into_pipe(pipe, *ppos, len); + } + + if (!n) + break; + len -= n; + total_spliced += n; + *ppos += n; + in->f_ra.prev_pos = *ppos; + if (pipe_full(pipe->head, pipe->tail, pipe->max_usage)) + break; + + cond_resched(); + } while (len); + + if (folio) + folio_put(folio); + + file_accessed(in); + return total_spliced ? total_spliced : error; +} + static loff_t shmem_file_llseek(struct file *file, loff_t offset, int whence) { struct address_space *mapping = file->f_mapping; @@ -3938,7 +4060,7 @@ static const struct file_operations shmem_file_operations = { .read_iter = shmem_file_read_iter, .write_iter = generic_file_write_iter, .fsync = noop_fsync, - .splice_read = generic_file_splice_read, + .splice_read = shmem_file_splice_read, .splice_write = iter_file_splice_write, .fallocate = shmem_fallocate, #endif