[PATCH 17/22] xfs: use shmem_get_folio in in xfile_load

Christoph Hellwig
2024-02-19T06:28:14Z

Switch to using shmem_get_folio in xfile_load instead of using
shmem_read_mapping_page_gfp. This gets us support for large folios
and also optimized reading from unallocated space, as
shmem_get_folio with SGP_READ won't allocate a page for them just
to zero the content.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Darrick J. Wong <djwong@kernel.org>
---
fs/xfs/scrub/xfile.c | 61 +++++++++++++++++++-------------------------
1 file changed, 26 insertions(+), 35 deletions(-)

diff --git a/fs/xfs/scrub/xfile.c b/fs/xfs/scrub/xfile.c
index a4480f4020ae16..2229f0b7f9ca49 100644
--- a/fs/xfs/scrub/xfile.c
+++ b/fs/xfs/scrub/xfile.c
@@ -34,13 +34,6 @@
* xfiles assume that the caller will handle all required concurrency
* management; standard vfs locks (freezer and inode) are not taken. Reads
* and writes are satisfied directly from the page cache.
- *
- * NOTE: The current shmemfs implementation has a quirk that in-kernel reads
- * of a hole cause a page to be mapped into the file. If you are going to
- * create a sparse xfile, please be careful about reading from uninitialized
- * parts of the file. These pages are !Uptodate and will eventually be
- * reclaimed if not written, but in the short term this boosts memory
- * consumption.
*/

/*
@@ -118,10 +111,7 @@ xfile_load(
loff_t pos)
{
struct inode *inode = file_inode(xf->file);
- struct address_space *mapping = inode->i_mapping;
- struct page *page = NULL;
unsigned int pflags;
- int error = 0;

if (count > MAX_RW_COUNT)
return -ENOMEM;
@@ -132,43 +122,44 @@ xfile_load(

pflags = memalloc_nofs_save();
while (count > 0) {
+ struct folio *folio;
unsigned int len;
+ unsigned int offset;

- len = min_t(ssize_t, count, PAGE_SIZE - offset_in_page(pos));
-
- /*
- * In-kernel reads of a shmem file cause it to allocate a page
- * if the mapping shows a hole. Therefore, if we hit ENOMEM
- * we can continue by zeroing the caller's buffer.
- */
- page = shmem_read_mapping_page_gfp(mapping, pos >> PAGE_SHIFT,
- __GFP_NOWARN);
- if (IS_ERR(page)) {
- error = PTR_ERR(page);
- if (error != -ENOMEM) {
- error = -ENOMEM;
+ if (shmem_get_folio(inode, pos >> PAGE_SHIFT, &folio,
+ SGP_READ) < 0)
+ break;
+ if (!folio) {
+ /*
+ * No data stored at this offset, just zero the output
+ * buffer until the next page boundary.
+ */
+ len = min_t(ssize_t, count,
+ PAGE_SIZE - offset_in_page(pos));
+ memset(buf, 0, len);
+ } else {
+ if (filemap_check_wb_err(inode->i_mapping, 0)) {
+ folio_unlock(folio);
+ folio_put(folio);
break;
}

- memset(buf, 0, len);
- goto advance;
- }
-
- /*
- * xfile pages must never be mapped into userspace, so
- * we skip the dcache flush.
- */
- memcpy(buf, page_address(page) + offset_in_page(pos), len);
- put_page(page);
+ offset = offset_in_folio(folio, pos);
+ len = min_t(ssize_t, count, folio_size(folio) - offset);
+ memcpy(buf, folio_address(folio) + offset, len);

-advance:
+ folio_unlock(folio);
+ folio_put(folio);
+ }
count -= len;
pos += len;
buf += len;
}
memalloc_nofs_restore(pflags);

- return error;
+ if (count)
+ return -ENOMEM;
+ return 0;
}

/*
--
2.39.2