dead/wrong code in ext3/4_releasepage()

Hello!

The implementation of ext4_releasepage() for many kernel versions (as well as current git) is the following:

static int ext4_releasepage(struct page *page, gfp_t wait)

{
        journal_t *journal = EXT4_JOURNAL(page->mapping->host);

        trace_ext4_releasepage(page);

        WARN_ON(PageChecked(page));
        if (!page_has_buffers(page))
                return 0;
        if (journal)
                return jbd2_journal_try_to_free_buffers(journal,
page, wait);
        else
                return try_to_free_buffers(page);
}

The "if (!page_has_buffers(page))" check seems to be attempting to handle the "nobh" case. However, the correct return value for this case seems to be 1 (success), not 0 (failure).

This does not lead to oom or any similar issue since calls to try_to_release_page()
are accompanied by page_has_private() checks.

If ->release_page() can be called without a prior check, then the return code is wrong. Otherwise, the check is dead code.

What do you think?

Quoted 5 lines · Andrew Perepechko

This does not lead to oom or any similar issue since calls to try_to_release_page()
are accompanied by page_has_private() checks.

The check is a dead code because ->release_page() is called only if
PagePrivate bit is set.

Honza

So you think the correct fix would be to remove the check completely since every try_to_release_page() caller is obliged to perform the page_has_private() check?

Quoted 10 lines · Andrew Perepechko

Quoted 5 lines

This does not lead to oom or any similar issue since calls to try_to_release_page()
are accompanied by page_has_private() checks.

The check is a dead code because ->release_page() is called only if
PagePrivate bit is set.

Honza

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Hi,

Quoted 3 lines · Andrew Perepechko

So you think the correct fix would be to remove the check completely since every try_to_release_page() caller is obliged to perform the page_has_private() check?

Yes, I think that would be reasonable. XFS already relies on this so
it should be safe.

Quoted 8 lines · Jan Kara

   Hi,

Quoted 3 lines · Andrew Perepechko

So you think the correct fix would be to remove the check completely since every try_to_release_page() caller is obliged to perform the page_has_private() check?

Yes, I think that would be reasonable. XFS already relies on this so
it should be safe.

Andrew, can you send me a patch?

Thanks,

- Ted
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