Re: [PATCH RFC 07/14] fs/erofs: mm/pagemap: add readahead_folio_reverse() to avoid folio->private

On Tue 04-08-26 13:09:46, Zi Yan wrote:
> On Tue Aug 4, 2026 at 1:04 PM EDT, Jan Kara wrote:
> > On Tue 04-08-26 11:54:41, Zi Yan wrote:
> >> On Tue Aug 4, 2026 at 5:32 AM EDT, Jan Kara wrote:
> >> > On Mon 03-08-26 12:56:36, Zi Yan wrote:
> >> >> On Mon Aug 3, 2026 at 5:54 AM EDT, Jan Kara wrote:
> >> >> > On Fri 31-07-26 22:13:30, Zi Yan wrote:
> >> >> >> erofs needs to traverse readahead folios in reverse order to achieve
> >> >> >> maximum performance by
> >> >> >> 1. reading all folios from readahead_folio();
> >> >> >> 2. storing the prior folio pointer in folio->private;
> >> >> >> 3. traverse from the last folio to the first one.
> >> >> >> 
> >> >> >> Add readahead_folio_reverse() to achieve the same function without using
> >> >> >> folio->private.
> >> >> >> 
> >> >> >> It prepares for a future commit that replaces PG_private checks with
> >> >> >> !folio->private checks. After switching the checks, erofs's use of
> >> >> >> folio->private without bumping folio refcount can cause unexpected
> >> >> >> outcomes, e.g., in filemap_release_folio(), try_to_free_buffers() becomes
> >> >> >> reachable.
> >> 
> >> <snip>
> >> 
> >> >> 
> >> >> The below is what I come up with. I did not add a bool to
> >> >> readahead_control, since I think that is the decision of caller of
> >> >> __readahead_advance(). But let me know if you disagree.
> >> >
> >> > The reason why I wanted bool in readahead_control is that if some code
> >> > ends up mixing readahead_folio() with readahead_folio_last() things will
> >> > get confused (because __readahead_advance() really wants to skip the batch
> >> > returned from the *previous* call to readahead_folio[_last]()). With the
> >> > bool in rac, even mixed use will properly advance the state of the
> >> > readahead_control. I don't think mixed use is very realistic (at this
> >> > point at least) so I'm ok with leaving that for later if you don't like it.
> >> 
> >> Got it. I am trying to figure out your mental model of how the mix of
> >> readahead_folio() and readahead_folio_last() works with the bool inside
> >> ractl. By looking at readahead_folio_last() code, it is almost the same
> >> as readahead_folio() with __readahead_folio() inlined
> >> (__readahead_folio() is only used by readahead_folio(), so the inline
> >> can happen without any issue). As a result, we can get rid of
> >> readahead_folio_last(), add set_readahead_direction() to set the
> >> embedded bool read_from_head, and use readahead_folio() only. This
> >> removes redundant code in readahead_folio_last(). One thing I am not
> >> certain is whether we want to
> >> 
> >> 1. use set_readahead_direction() explicit and warn readahead_folio() if
> >> read_from_head is not initialized, or
> >> 
> >> 2. set read_from_head to true by default, so that only erofs needs to
> >> call set_readahead_direction() to change read_from_head.
> >> 
> >> The former is less confusing but changes how readahead_folio() works;
> >> the latter is simpler but implicit read_from_head state might confuse
> >> people at some point.
> >
> > My idea was: readahead_folio() will call __readahead_advance() and then set
> > rac->forward = true. readahead_folio_last() will call __readahead_advance()
> > and set rac->forward = false. __readahead_advance() advances from beginning
> > / end based on rac->_forward value.
> 
> Got it. I can do that. Just to be clear, it should be that
> readahead_folio() first sets rac->forward = true, then calls
> __readahead_advance(), since __readahead_advance() advances based on
> rac->forward, right? readahead_folio_last() as well.

No. I wrote "and then set" which means after and that is what I really
wanted to say. You still don't seem to be understanding the logic of handling
the _batch_count. _batch_count is the length of the returned batch.
__readahead_advance() updates _index and _nr_pages to remove the folios
returned in the last batch from the range. So _forward needs to contain
whether the last returned batch was taken from the beginning or the end of
the range and __readahead_advance() uses it to update current range
accordingly (before we go and return the next batch). We cannot clobber
_forward before calling __readahead_advance(). I hope things are clearer
now.

								Honza
-- 
Jan Kara <[email protected]>
SUSE Labs, CR