On Mon, Jun 29, 2020 at 11:24:14AM -0700, Jaegeuk Kim wrote: > On 06/29, Eric Biggers wrote: > > 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. > > I just wanted to keep zero overhead, since it doesn't matter either cases of > skipping readahead or not. >
Okay, then it should use READ_ONCE() and WRITE_ONCE(). - Eric
