On Mon, Jun 29, 2020 at 08:03:23AM -0700, Jaegeuk Kim wrote: > If two readahead threads having same offset enter in readpages, every read > IOs are split and issued to the disk which giving lower bandwidth. > > This patch tries to avoid redundant readahead calls. > > Signed-off-by: Jaegeuk Kim <[email protected]> > --- > v2: > - add missing code to bypass read > > fs/f2fs/data.c | 18 +++++++++++++++++- > fs/f2fs/f2fs.h | 1 + > fs/f2fs/super.c | 2 ++ > 3 files changed, 20 insertions(+), 1 deletion(-) > > diff --git a/fs/f2fs/data.c b/fs/f2fs/data.c > index d6094b9f3916..9b69a159cc6c 100644 > --- a/fs/f2fs/data.c > +++ b/fs/f2fs/data.c > @@ -2403,6 +2403,7 @@ int f2fs_mpage_readpages(struct address_space *mapping, > #endif > unsigned max_nr_pages = nr_pages; > int ret = 0; > + bool drop_ra = false; > > map.m_pblk = 0; > map.m_lblk = 0; > @@ -2413,13 +2414,25 @@ int f2fs_mpage_readpages(struct address_space > *mapping, > map.m_seg_type = NO_CHECK_TYPE; > map.m_may_create = false; > > + /* > + * Two readahead threads for same address range can cause race condition > + * which fragments sequential read IOs. So let's avoid each other. > + */ > + if (pages && is_readahead) { > + page = list_last_entry(pages, struct page, lru); > + if (F2FS_I(inode)->ra_offset == page_index(page)) > + drop_ra = true; > + else > + F2FS_I(inode)->ra_offset = page_index(page); > + }
This is a data race because ra_offset can be read/written by different threads concurrently. It either needs locking, or READ_ONCE() and WRITE_ONCE() if races are okay. - Eric
