[PATCH v2 1/3] fs: make the i_size_read/write helpers be smp_load_acquire/store_release()

From: Baokun Li
Date: Wed Jan 24 2024 - 09:26:58 EST


In [Link] Linus mentions that acquire/release makes it clear which
_particular_ memory accesses are the ordered ones, and it's unlikely
to make any performance difference, so it's much better to pair up
the release->acquire ordering than have a "wmb->rmb" ordering.

=========================================================
update pagecache
folio_mark_uptodate(folio)
smp_wmb()
set_bit PG_uptodate

=== ↑↑↑ STLR ↑↑↑ === smp_store_release(&inode->i_size, i_size)

folio_test_uptodate(folio)
test_bit PG_uptodate
smp_rmb()

=== ↓↓↓ LDAR ↓↓↓ === smp_load_acquire(&inode->i_size)

copy_page_to_iter()
=========================================================

Calling smp_store_release() in i_size_write() ensures that the data
in the page and the PG_uptodate bit are updated before the isize is
updated, and calling smp_load_acquire() in i_size_read ensures that
it will not read a newer isize than the data in the page. Therefore,
this avoids buffered read-write inconsistencies caused by Load-Load
reordering.

Link: https://lore.kernel.org/r/CAHk-=wifOnmeJq+sn+2s-P46zw0SFEbw9BSCGgp2c5fYPtRPGw@xxxxxxxxxxxxxx/
Suggested-by: Linus Torvalds <torvalds@xxxxxxxxxxxxxxxxxxxx>
Signed-off-by: Baokun Li <libaokun1@xxxxxxxxxx>
---
include/linux/fs.h | 10 ++++++++--
1 file changed, 8 insertions(+), 2 deletions(-)

diff --git a/include/linux/fs.h b/include/linux/fs.h
index 6bb10bbd7035..1cc1f3f08107 100644
--- a/include/linux/fs.h
+++ b/include/linux/fs.h
@@ -907,7 +907,8 @@ static inline loff_t i_size_read(const struct inode *inode)
preempt_enable();
return i_size;
#else
- return inode->i_size;
+ /* Pairs with smp_store_release() in i_size_write() */
+ return smp_load_acquire(&inode->i_size);
#endif
}

@@ -929,7 +930,12 @@ static inline void i_size_write(struct inode *inode, loff_t i_size)
inode->i_size = i_size;
preempt_enable();
#else
- inode->i_size = i_size;
+ /*
+ * Pairs with smp_load_acquire() in i_size_read() to ensure
+ * changes related to inode size (such as page contents) are
+ * visible before we see the changed inode size.
+ */
+ smp_store_release(&inode->i_size, i_size);
#endif
}

--
2.31.1