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fs: make the i_size_read/write helpers be smp_load_acquire/store_release()
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@mail.gmail.com/ Suggested-by: Linus Torvalds <torvalds@linux-foundation.org> Signed-off-by: Baokun Li <libaokun1@huawei.com> Link: https://lore.kernel.org/r/20240124142857.4146716-2-libaokun1@huawei.com Signed-off-by: Christian Brauner <brauner@kernel.org>
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1 changed files with 8 additions and 2 deletions
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@ -907,7 +907,8 @@ static inline loff_t i_size_read(const struct inode *inode)
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preempt_enable();
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return i_size;
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#else
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return inode->i_size;
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/* Pairs with smp_store_release() in i_size_write() */
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return smp_load_acquire(&inode->i_size);
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#endif
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}
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@ -929,7 +930,12 @@ static inline void i_size_write(struct inode *inode, loff_t i_size)
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inode->i_size = i_size;
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preempt_enable();
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#else
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inode->i_size = i_size;
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/*
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* Pairs with smp_load_acquire() in i_size_read() to ensure
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* changes related to inode size (such as page contents) are
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* visible before we see the changed inode size.
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*/
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smp_store_release(&inode->i_size, i_size);
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#endif
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}
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