Re: [QUESTION] xas_reload() in iter_xarray_populate_pages()

[David Hildenbrand]

On 17.06.25 11:18, Dev Jain wrote:
> On 17/06/25 1:17 pm, David Hildenbrand wrote:
> > On 17.06.25 07:10, Dev Jain wrote:
> > > On 26/05/25 12:05 pm, Dev Jain wrote:
> > > > Hello all,
> > > >
> > > > After doing an xas_load() and xas_retry(), we take neither a
> > > > reference nor a lock
> > > > on the folio, and we do an xas_reload(). Is this just to reduce the
> > > > time window
> > > > for a race?
> > > >
> > > > If the above is true, then, there is a negligible window between
> > > > xas_load() and
> > > > xas_reload(), because only xas_retry() exists between them, so why
> > > > to even reload()?
> > > >
> > > > Thanks,
> > > > Dev
> > >
> > > I do not completely remember our discussion in THP Cabal; I recall
> > > David Howells maybe
> > > saying that the folios are already locked, so it is safe to do
> > > xas_load and then do
> > > a folio_get()? Even if we remove the redundant xas_reload(), I still
> > > don't understand
> > > why we won't need xas_reload() at least after folio_get()?
> >
> > I think the points where
> >
> > (a) this should go all away soon
> >
> > (b) there is the expectation that the folios cannot get truncated
> >      concurrently. So we can do an unconditional folio_get(), don't have
> >      to check folio->mapping etc.
>
> Well...pretty sure the file read path is taking the inode->i_rwsem or
> i_lock somewhere,
>
> to synchronize with truncation/reclaim, can't figure out where. Reclaim
> takes the i_lock in __remove_mapping and
>
> then freezes the folio reference, so the read path must lock i_lock
> somewhere.

I mean, if concurrent freeing of a folio would be possible, the function 
would be horribly broken :)

--
Cheers,

David / dhildenb