Linus Torvalds <torvalds@xxxxxxxxxxxxxxxxxxxx> wrote:
> But I also think it's strange in another way, with that odd placement of
>
> mapping_clear_release_always(inode->i_mapping);
>
> at inode eviction time. That just feels very random.
I was under the impression that a warning got splashed if unexpected
address_space flags were set when ->evict_inode() returned. I may be thinking
of page flags. If it doesn't, fine, this isn't required.
> Similarly, that change to shrink_folio_list() looks strange, with the
> nasty folio_needs_release() helper. It seems entirely pointless, with
> the use then being
>
> if (folio_needs_release(folio)) {
> if (!filemap_release_folio(folio, sc->gfp_mask))
> goto activate_locked;
Unfortunately, that can't be simply folded down. It actually does something
extra if folio_has_private() was set, filemap_release_folio() succeeds but
there was no mapping:
* Rarely, folios can have buffers and no ->mapping.
* These are the folios which were not successfully
* invalidated in truncate_cleanup_folio(). We try to
* drop those buffers here and if that worked, and the
* folio is no longer mapped into process address space
* (refcount == 1) it can be freed. Otherwise, leave
* the folio on the LRU so it is swappable.
Possibly I could split the if-statement and make it two separate cases:
/*
* If the folio has buffers, try to free the buffer
* mappings associated with this folio. If we succeed
* we try to free the folio as well.
*
* We do this even if the folio is dirty.
* filemap_release_folio() does not perform I/O, but it
* is possible for a folio to have the dirty flag set,
* but it is actually clean (all its buffers are clean).
* This happens if the buffers were written out directly,
* with submit_bh(). ext3 will do this, as well as
* the blockdev mapping. filemap_release_folio() will
* discover that cleanness and will drop the buffers
* and mark the folio clean - it can be freed.
*/
if (!filemap_release_folio(folio, sc->gfp_mask))
goto activate_locked;
filemap_release_folio() will return true if folio_has_private() is false,
which would allow us to reach the next part, which we would then skip.
/*
* Rarely, folios can have buffers and no ->mapping.
* These are the folios which were not successfully
* invalidated in truncate_cleanup_folio(). We try to
* drop those buffers here and if that worked, and the
* folio is no longer mapped into process address space
* (refcount == 1) it can be freed. Otherwise, leave
* the folio on the LRU so it is swappable.
*/
if (!mapping && folio_has_private(folio) &&
folio_ref_count(folio) == 1) {
folio_unlock(folio);
if (folio_put_testzero(folio))
goto free_it;
/*
* rare race with speculative reference.
* the speculative reference will free
* this folio shortly, so we may
* increment nr_reclaimed here (and
* leave it off the LRU).
*/
nr_reclaimed += nr_pages;
continue;
}
But that will malfunction if try_to_free_buffers(), as called from
folio_has_private(), manages to clear the private bits. I wonder if it might
be possible to fold this bit into filemap_release_folio() somehow.
I really need a three-state return from filemap_release_folio() - maybe:
0 couldn't release
1 released
2 there was no private
The ordinary "if (filemap_release_folio()) { ... }" would work as expected.
shrink_folio_list() could do something different between case 1 and case 2.
> And the change to mm/filemap.c is completely unacceptable in all
> forms, and this added test
>
> + if ((!mapping || !mapping_release_always(mapping)) &&
> + !folio_test_private(folio) &&
> + !folio_test_private_2(folio))
> + return true;
>
> will not be accepted even during the merge window. That code makes no
> sense what-so-ever, and is in no way acceptable.
>
> That code makes no sense what-so-ever. Why isn't it using
> "folio_has_private()"?
It should be, yes.
> Why is this done as an open-coded - and *badly* so - version of
> !folio_needs_release() that you for some reason made private to mm/vmscan.c?
Yeah, in retrospect, I should have put that in mm/internal.h.
David
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