[PATCH v3 2/2] fs: generic_file_buffered_read() now uses find_get_pages_contig

Kent Overstreet
2020-06-19T03:21:07Z

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 <kent.overstreet@gmail.com>
---
mm/filemap.c | 279 +++++++++++++++++++++++++++++----------------------
1 file changed, 159 insertions(+), 120 deletions(-)

diff --git a/mm/filemap.c b/mm/filemap.c
index dc4a72042e..d8bd5e9647 100644
--- a/mm/filemap.c
+++ b/mm/filemap.c
@@ -1975,67 +1975,6 @@ static void shrink_readahead_size_eio(struct file_ra_state *ra)
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 int offset = iocb->ki_pos & ~PAGE_MASK;
- unsigned int 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,
@@ -2179,6 +2118,83 @@ 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 int 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, nr_got, err = 0;
+
+ nr = min_t(unsigned long, last_index - index, nr);
+find_page:
+ if (fatal_signal_pending(current))
+ return -EINTR;
+
+ nr_got = find_get_pages_contig(mapping, index, nr, pages);
+ if (nr_got)
+ goto got_pages;
+
+ if (iocb->ki_flags & IOCB_NOWAIT)
+ return -EAGAIN;
+
+ page_cache_sync_readahead(mapping, ra, filp, index, last_index - index);
+
+ nr_got = find_get_pages_contig(mapping, index, nr, pages);
+ if (nr_got)
+ goto got_pages;
+
+ pages[0] = generic_file_buffered_read_no_cached_page(iocb, iter);
+ err = PTR_ERR_OR_ZERO(pages[0]);
+ if (!IS_ERR_OR_NULL(pages[0]))
+ nr_got = 1;
+got_pages:
+ for (i = 0; i < nr_got; 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 < nr_got; j++)
+ put_page(pages[j]);
+ nr_got = 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 < nr_got; j++)
+ put_page(pages[j]);
+ nr_got = i;
+ err = PTR_ERR_OR_ZERO(page);
+ break;
+ }
+ }
+ }
+
+ if (likely(nr_got))
+ return nr_got;
+ if (err)
+ return err;
+ /*
+ * No pages and no error means we raced and should retry:
+ */
+ goto find_page;
+}
+
/**
* generic_file_buffered_read - generic file read routine
* @iocb: the iocb to read
@@ -2199,86 +2215,109 @@ 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_onstack[8], **pages = NULL;
+ unsigned int nr_pages = ((iocb->ki_pos + iter->count + PAGE_SIZE - 1) >> PAGE_SHIFT) -
+ (iocb->ki_pos >> PAGE_SHIFT);
+ 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;
+ if (nr_pages > ARRAY_SIZE(pages_onstack))
+ pages = kmalloc_array(nr_pages, sizeof(void *), GFP_KERNEL);

- for (;;) {
- pgoff_t index = iocb->ki_pos >> PAGE_SHIFT;
- struct page *page;
+ if (!pages) {
+ pages = pages_onstack;
+ nr_pages = min_t(unsigned int, nr_pages, ARRAY_SIZE(pages_onstack));
+ }

+ 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, nr_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 int offset = iocb->ki_pos & ~PAGE_MASK;
+ unsigned int bytes = min_t(loff_t, end_offset - iocb->ki_pos,
+ PAGE_SIZE - offset);
+ unsigned int 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(pages[i]);
+
+ 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);

+ if (pages != pages_onstack)
+ kfree(pages);
+
return written ? written : error;
}

--
2.26.2