From: Qian Yingjin qian@ddn.com

I was running traces of the read code against an RAID storage system to understand why read requests were being misaligned against the underlying RAID strips. I found that the page end offset calculation in filemap_get_read_batch() was off by one.

When a read is submitted with end offset 1048575, then it calculates the end page for read of 256 when it should be 255. "last_index" is the index of the page beyond the end of the read and it should be skipped when get a batch of pages for read in @filemap_get_read_batch().

The below simple patch fixes the problem. This code was introduced in kernel 5.12.

Fixes: cbd59c48ae2b ("mm/filemap: use head pages in generic_file_buffered_read")

Signed-off-by: Qian Yingjin qian@ddn.com --- mm/filemap.c | 9 +++++++-- 1 file changed, 7 insertions(+), 2 deletions(-)

diff --git a/mm/filemap.c b/mm/filemap.c index c4d4ace9cc70..b7754760c09a 100644 --- a/mm/filemap.c +++ b/mm/filemap.c @@ -2371,7 +2371,7 @@ static void shrink_readahead_size_eio(struct file_ra_state *ra) * clear so that the caller can take the appropriate action. */ static void filemap_get_read_batch(struct address_space *mapping, - pgoff_t index, pgoff_t max, struct folio_batch *fbatch) + pgoff_t index, pgoff_t last_index, struct folio_batch *fbatch) { XA_STATE(xas, &mapping->i_pages, index); struct folio *folio; @@ -2380,7 +2380,11 @@ static void filemap_get_read_batch(struct address_space *mapping, for (folio = xas_load(&xas); folio; folio = xas_next(&xas)) { if (xas_retry(&xas, folio)) continue; - if (xas.xa_index > max || xa_is_value(folio)) + /* + * "last_index" is the index of the page beyond the end of + * the read. + */ + if (xas.xa_index >= last_index || xa_is_value(folio)) break; if (xa_is_sibling(folio)) break; @@ -2588,6 +2592,7 @@ static int filemap_get_pages(struct kiocb *iocb, struct iov_iter *iter, struct folio *folio; int err = 0;

+ /* "last_index" is the index of the page beyond the end of the read */ last_index = DIV_ROUND_UP(iocb->ki_pos + iter->count, PAGE_SIZE); retry: if (fatal_signal_pending(current))