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[71.234.172.214]) by smtp.gmail.com with ESMTPSA id h10-v6sm6204361ywa.35.2018.08.15.16.26.39 (version=TLS1_2 cipher=ECDHE-RSA-AES128-GCM-SHA256 bits=128/128); Wed, 15 Aug 2018 16:26:40 -0700 (PDT) From: Kent Overstreet To: linux-kernel@vger.kernel.org, linux-fsdevel@vger.kernel.org, viro@zeniv.linux.org.uk Cc: Kent Overstreet Subject: [PATCH 2/2] fs: generic_file_buffered_read() now uses find_get_pages_contig Date: Wed, 15 Aug 2018 19:26:32 -0400 Message-Id: <20180815232632.32548-3-kent.overstreet@gmail.com> X-Mailer: git-send-email 2.18.0 In-Reply-To: <20180815232632.32548-1-kent.overstreet@gmail.com> References: <20180815232632.32548-1-kent.overstreet@gmail.com> Sender: linux-kernel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Convert generic_file_buffered_read() to get pages to read from in batches, and then copy data to userspace from many pages at once - in particular, we now don't touch any cachelines that might be contended while we're in the loop to copy data to userspace. This is is a performance improvement on workloads that do buffered reads with large blocksizes, and a very large performance improvement if that file is also being accessed concurrently by different threads. On smaller reads (512 bytes), there's a very small performance improvement (1%, within the margin of error). Signed-off-by: Kent Overstreet --- mm/filemap.c | 266 ++++++++++++++++++++++++++++----------------------- 1 file changed, 144 insertions(+), 122 deletions(-) diff --git a/mm/filemap.c b/mm/filemap.c index 308bdd466f..0de9fe4c61 100644 --- a/mm/filemap.c +++ b/mm/filemap.c @@ -2110,67 +2110,6 @@ static void shrink_readahead_size_eio(struct file *filp, ra->ra_pages /= 4; } -static int generic_file_buffered_read_page_ok(struct kiocb *iocb, - struct iov_iter *iter, - struct page *page) -{ - struct address_space *mapping = iocb->ki_filp->f_mapping; - struct inode *inode = mapping->host; - struct file_ra_state *ra = &iocb->ki_filp->f_ra; - unsigned offset = iocb->ki_pos & ~PAGE_MASK; - unsigned bytes, copied; - loff_t isize, end_offset; - - BUG_ON(iocb->ki_pos >> PAGE_SHIFT != page->index); - - /* - * i_size must be checked after we know the page is Uptodate. - * - * Checking i_size after the check allows us to calculate - * the correct value for "bytes", 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(iocb->ki_pos >= isize)) - return 1; - - end_offset = min_t(loff_t, isize, iocb->ki_pos + iter->count); - - bytes = min_t(loff_t, end_offset - iocb->ki_pos, PAGE_SIZE - offset); - - /* 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_page(page); - - /* - * Ok, we have the page, and it's up-to-date, so - * now we can copy it to user space... - */ - - copied = copy_page_to_iter(page, offset, bytes, iter); - - iocb->ki_pos += copied; - - /* - * When a sequential read accesses a page several times, - * only mark it as accessed the first time. - */ - if (iocb->ki_pos >> PAGE_SHIFT != ra->prev_pos >> PAGE_SHIFT) - mark_page_accessed(page); - - ra->prev_pos = iocb->ki_pos; - - if (copied < bytes) - return -EFAULT; - - return !iov_iter_count(iter) || iocb->ki_pos == isize; -} - static struct page * generic_file_buffered_read_readpage(struct file *filp, struct address_space *mapping, @@ -2314,6 +2253,79 @@ generic_file_buffered_read_no_cached_page(struct kiocb *iocb, return generic_file_buffered_read_readpage(filp, mapping, page); } +static int generic_file_buffered_read_get_pages(struct kiocb *iocb, + struct iov_iter *iter, + struct page **pages, + unsigned nr) +{ + struct file *filp = iocb->ki_filp; + struct address_space *mapping = filp->f_mapping; + struct file_ra_state *ra = &filp->f_ra; + pgoff_t index = iocb->ki_pos >> PAGE_SHIFT; + pgoff_t last_index = (iocb->ki_pos + iter->count + PAGE_SIZE-1) >> PAGE_SHIFT; + int i, j, ret, err = 0; + + nr = min_t(unsigned long, last_index - index, nr); +find_page: + if (fatal_signal_pending(current)) + return -EINTR; + + ret = find_get_pages_contig(mapping, index, nr, pages); + if (ret) + goto got_pages; + + if (iocb->ki_flags & IOCB_NOWAIT) + return -EAGAIN; + + page_cache_sync_readahead(mapping, ra, filp, index, last_index - index); + + ret = find_get_pages_contig(mapping, index, nr, pages); + if (ret) + goto got_pages; + + pages[0] = generic_file_buffered_read_no_cached_page(iocb, iter); + err = PTR_ERR_OR_ZERO(pages[0]); + ret = !IS_ERR_OR_NULL(pages[0]); +got_pages: + for (i = 0; i < ret; i++) { + struct page *page = pages[i]; + pgoff_t pg_index = index +i; + loff_t pg_pos = max(iocb->ki_pos, + (loff_t) pg_index << PAGE_SHIFT); + loff_t pg_count = iocb->ki_pos + iter->count - pg_pos; + + if (PageReadahead(page)) + page_cache_async_readahead(mapping, ra, filp, page, + pg_index, last_index - pg_index); + + if (!PageUptodate(page)) { + if (iocb->ki_flags & IOCB_NOWAIT) { + for (j = i; j < ret; j++) + put_page(pages[j]); + ret = i; + err = -EAGAIN; + break; + } + + page = generic_file_buffered_read_pagenotuptodate(filp, + iter, page, pg_pos, pg_count); + if (IS_ERR_OR_NULL(page)) { + for (j = i + 1; j < ret; j++) + put_page(pages[j]); + ret = i; + err = PTR_ERR_OR_ZERO(page); + break; + } + } + } + + if (likely(ret)) + return ret; + if (err) + return err; + goto find_page; +} + /** * generic_file_buffered_read - generic file read routine * @iocb: the iocb to read @@ -2330,83 +2342,93 @@ static ssize_t generic_file_buffered_read(struct kiocb *iocb, struct iov_iter *iter, ssize_t written) { struct file *filp = iocb->ki_filp; + struct file_ra_state *ra = &filp->f_ra; struct address_space *mapping = filp->f_mapping; struct inode *inode = mapping->host; - struct file_ra_state *ra = &filp->f_ra; size_t orig_count = iov_iter_count(iter); - pgoff_t last_index; - int error = 0; + struct page *pages[64]; + int i, pg_nr, error = 0; + bool writably_mapped; + loff_t isize, end_offset; if (unlikely(iocb->ki_pos >= inode->i_sb->s_maxbytes)) return 0; iov_iter_truncate(iter, inode->i_sb->s_maxbytes); - last_index = (iocb->ki_pos + iter->count + PAGE_SIZE-1) >> PAGE_SHIFT; - - for (;;) { - pgoff_t index = iocb->ki_pos >> PAGE_SHIFT; - struct page *page; - + do { cond_resched(); -find_page: - if (fatal_signal_pending(current)) { - error = -EINTR; - goto out; - } - page = find_get_page(mapping, index); - if (!page) { - if (iocb->ki_flags & IOCB_NOWAIT) - goto would_block; - page_cache_sync_readahead(mapping, - ra, filp, - index, last_index - index); - page = find_get_page(mapping, index); - if (unlikely(page == NULL)) { - page = generic_file_buffered_read_no_cached_page(iocb, iter); - if (!page) - goto find_page; - if (IS_ERR(page)) { - error = PTR_ERR(page); - goto out; - } - } - } - if (PageReadahead(page)) { - page_cache_async_readahead(mapping, - ra, filp, page, - index, last_index - index); + i = 0; + pg_nr = generic_file_buffered_read_get_pages(iocb, iter, pages, + ARRAY_SIZE(pages)); + if (pg_nr < 0) { + error = pg_nr; + break; } - if (!PageUptodate(page)) { - if (iocb->ki_flags & IOCB_NOWAIT) { - put_page(page); - error = -EAGAIN; - goto out; - } - page = generic_file_buffered_read_pagenotuptodate(filp, - iter, page, iocb->ki_pos, iter->count); - if (!page) - goto find_page; - if (IS_ERR(page)) { - error = PTR_ERR(page); - goto out; - } - } + /* + * 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(iocb->ki_pos >= isize)) + goto put_pages; - error = generic_file_buffered_read_page_ok(iocb, iter, page); - put_page(page); + end_offset = min_t(loff_t, isize, iocb->ki_pos + iter->count); - if (error) { - if (error > 0) - error = 0; - goto out; + while ((iocb->ki_pos >> PAGE_SHIFT) + pg_nr > + (end_offset + PAGE_SIZE - 1) >> PAGE_SHIFT) + put_page(pages[--pg_nr]); + + /* + * Once we start copying data, we don't want to be touching any + * cachelines that might be contended: + */ + writably_mapped = mapping_writably_mapped(mapping); + + /* + * When a sequential read accesses a page several times, only + * mark it as accessed the first time. + */ + if (iocb->ki_pos >> PAGE_SHIFT != + ra->prev_pos >> PAGE_SHIFT) + mark_page_accessed(pages[0]); + for (i = 1; i < pg_nr; i++) + mark_page_accessed(pages[i]); + + for (i = 0; i < pg_nr; i++) { + unsigned offset = iocb->ki_pos & ~PAGE_MASK; + unsigned bytes = min_t(loff_t, end_offset - iocb->ki_pos, + PAGE_SIZE - offset); + unsigned copied; + + /* + * 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 (writably_mapped) + flush_dcache_page(page); + + copied = copy_page_to_iter(pages[i], offset, bytes, iter); + + iocb->ki_pos += copied; + ra->prev_pos = iocb->ki_pos; + + if (copied < bytes) { + error = -EFAULT; + break; + } } - } +put_pages: + for (i = 0; i < pg_nr; i++) + put_page(pages[i]); + } while (iov_iter_count(iter) && iocb->ki_pos < isize && !error); -would_block: - error = -EAGAIN; -out: file_accessed(filp); written += orig_count - iov_iter_count(iter); -- 2.18.0