Re: [PATCH v3 6/8] filemap: Allow __filemap_get_folio to allocate large folios

[Matthew Wilcox <willy@infradead.org>]

On Tue, Jun 13, 2023 at 08:49:05AM +1000, Dave Chinner wrote:
> On Mon, Jun 12, 2023 at 09:39:08PM +0100, Matthew Wilcox (Oracle) wrote:
> > Allow callers of __filemap_get_folio() to specify a preferred folio
> > order in the FGP flags.  This is only honoured in the FGP_CREATE path;
> > if there is already a folio in the page cache that covers the index,
> > we will return it, no matter what its order is.  No create-around is
> > attempted; we will only create folios which start at the specified index.
> > Unmodified callers will continue to allocate order 0 folios.
> .....
> > -		/* Init accessed so avoid atomic mark_page_accessed later */
> > -		if (fgp_flags & FGP_ACCESSED)
> > -			__folio_set_referenced(folio);
> > +		if (!mapping_large_folio_support(mapping))
> > +			order = 0;
> > +		if (order > MAX_PAGECACHE_ORDER)
> > +			order = MAX_PAGECACHE_ORDER;
> > +		/* If we're not aligned, allocate a smaller folio */
> > +		if (index & ((1UL << order) - 1))
> > +			order = __ffs(index);
> 
> If I read this right, if we pass in an unaligned index, we won't get
> the size of the folio we ask for?

Right.  That's implied by (but perhaps not obvious from) the changelog.
Folios are always naturally aligned in the file, so an order-4 folio
has to start at a multiple of 16.  If the index you pass in is not
a multiple of 16, we can't create an order-4 folio without starting
at an earlier index.

For a 4kB block size filesystem, that's what we want.  Applications
_generally_ don't write backwards, so creating an order-4 folio is just
wasting memory.

> e.g. if we want an order-4 folio (64kB) because we have a 64kB block
> size in the filesystem, then we have to pass in an index that
> order-4 aligned, yes?
> 
> I ask this, because the later iomap code that asks for large folios
> only passes in "pos >> PAGE_SHIFT" so it looks to me like it won't
> allocate large folios for anything other than large folio aligned
> writes, even if we need them.
> 
> What am I missing?

Perhaps what you're missing is that this isn't trying to solve the
problem of supporting a bs > ps filesystem?  That's also a worthwhile
project, but it's not this project.  In fact, I'd say that project is
almost orthogonal to this one; for this usage we can always fall back to
smaller folios on memory pressure or misalignment.  For a bs > ps block
device, we have to allocate folios at least as large as the blocksize
and cannot fall back to smaller folios.  For a bs > ps filesystem on a
bdev with bs == ps, we can fall back (as your prototype showed).