mm: shmem: update iocb->ki_pos directly to simplify tmpfs read logic

Patch series "Improve the tmpfs large folio read performance", v2.

tmpfs already supports PMD-sized large folios, but the tmpfs read
operation still performs copying at PAGE_SIZE granularity, which is not
perfect.  This patchset changes tmpfs to copy data at the folio
granularity, which can improve the read performance.

Use 'fio bs=64k' to read a 1G tmpfs file populated with 2M THPs, and I can
see about 20% performance improvement, and no regression with bs=4k.  I
also did some functional testing with the xfstests suite, and I did not
find any regressions with the following xfstests config:

  FSTYP=tmpfs
  export TEST_DIR=/mnt/tempfs_mnt
  export TEST_DEV=/mnt/tempfs_mnt
  export SCRATCH_MNT=/mnt/scratchdir
  export SCRATCH_DEV=/mnt/scratchdir

This patch (of 2):

Using iocb->ki_pos to check if the read bytes exceeds the file size and to
calculate the bytes to be read can help simplify the code logic.
Meanwhile, this is also a preparation for improving tmpfs large folios
read performance in the following patch.

Link: https://lkml.kernel.org/r/cover.1729218573.git.baolin.wang@linux.alibaba.com
Link: https://lkml.kernel.org/r/e8863e289577e0dc1e365b5419bf2d1c9a24ae3d.1729218573.git.baolin.wang@linux.alibaba.com
Signed-off-by: Baolin Wang <baolin.wang@linux.alibaba.com>
Reviewed-by: Yang Shi <shy828301@gmail.com>
Cc: David Hildenbrand <david@redhat.com>
Cc: Hugh Dickins <hughd@google.com>
Cc: Kefeng Wang <wangkefeng.wang@huawei.com>
Cc: Matthew Wilcox <willy@infradead.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

diff --git a/mm/shmem.c b/mm/shmem.c
index ebf39aa0b9ab57757a3e0439e4ed5744ccc10327..26aaddc52fd1da4b834f13d9f0c245ba887663f2 100644 (file)
--- a/mm/shmem.c
+++ b/mm/shmem.c
@@ -3093,27 +3093,19 @@ static ssize_t shmem_file_read_iter(struct kiocb *iocb, struct iov_iter *to)
        unsigned long offset;
        int error = 0;
        ssize_t retval = 0;
-       loff_t *ppos = &iocb->ki_pos;
 
-       index = *ppos >> PAGE_SHIFT;
-       offset = *ppos & ~PAGE_MASK;
+       offset = iocb->ki_pos & ~PAGE_MASK;
 
        for (;;) {
                struct folio *folio = NULL;
                struct page *page = NULL;
-               pgoff_t end_index;
                unsigned long nr, ret;
-               loff_t i_size = i_size_read(inode);
+               loff_t end_offset, i_size = i_size_read(inode);
 
-               end_index = i_size >> PAGE_SHIFT;
-               if (index > end_index)
+               if (unlikely(iocb->ki_pos >= i_size))
                        break;
-               if (index == end_index) {
-                       nr = i_size & ~PAGE_MASK;
-                       if (nr <= offset)
-                               break;
-               }
 
+               index = iocb->ki_pos >> PAGE_SHIFT;
                error = shmem_get_folio(inode, index, 0, &folio, SGP_READ);
                if (error) {
                        if (error == -EINVAL)
@@ -3135,18 +3127,14 @@ static ssize_t shmem_file_read_iter(struct kiocb *iocb, struct iov_iter *to)
                 * We must evaluate after, since reads (unlike writes)
                 * are called without i_rwsem protection against truncate
                 */
-               nr = PAGE_SIZE;
                i_size = i_size_read(inode);
-               end_index = i_size >> PAGE_SHIFT;
-               if (index == end_index) {
-                       nr = i_size & ~PAGE_MASK;
-                       if (nr <= offset) {
-                               if (folio)
-                                       folio_put(folio);
-                               break;
-                       }
+               if (unlikely(iocb->ki_pos >= i_size)) {
+                       if (folio)
+                               folio_put(folio);
+                       break;
                }
-               nr -= offset;
+               end_offset = min_t(loff_t, i_size, iocb->ki_pos + to->count);
+               nr = min_t(loff_t, end_offset - iocb->ki_pos, PAGE_SIZE - offset);
 
                if (folio) {
                        /*
@@ -3186,8 +3174,8 @@ static ssize_t shmem_file_read_iter(struct kiocb *iocb, struct iov_iter *to)
 
                retval += ret;
                offset += ret;
-               index += offset >> PAGE_SHIFT;
                offset &= ~PAGE_MASK;
+               iocb->ki_pos += ret;
 
                if (!iov_iter_count(to))
                        break;
@@ -3198,7 +3186,6 @@ static ssize_t shmem_file_read_iter(struct kiocb *iocb, struct iov_iter *to)
                cond_resched();
        }
 
-       *ppos = ((loff_t) index << PAGE_SHIFT) + offset;
        file_accessed(file);
        return retval ? retval : error;
 }