Re: [f2fs-dev] [PATCH 0/4] f2fs: Remove uses of writepage

[Matthew Wilcox <willy@infradead.org>]

On Fri, Mar 14, 2025 at 09:38:15PM +0000, Jaegeuk Kim wrote:
> On 03/14, Matthew Wilcox wrote:
> > Unfortunately, I thnk I have to abandon this effort.  It's only going
> > to make supporting large folios harder (ie there would then need to be
> > an equivalently disruptive series adding support for large folios).
> > 
> > The fundamental problem is that f2fs has no concept of block size !=
> > PAGE_SIZE.  So if you create a filesystem on a 4kB PAGE_SIZE kernel,
> > you can't mount it on a 16kB PAGE_SIZE kernel.  An example:
> > 
> > int f2fs_recover_inline_xattr(struct inode *inode, struct page *page)
> > {
> >         struct f2fs_inode *ri;
> >         ipage = f2fs_get_node_page(F2FS_I_SB(inode), inode->i_ino);
> >         ri = F2FS_INODE(page);
> > 
> > so an inode number is an index into the filesystem in PAGE_SIZE units,
> > not in filesystem block size units.  Fixing this is a major effort, and
> > I lack the confidence in my abilities to do it without breaking anything.
> > 
> > As an outline of what needs to happen, I think that rather than passing
> > around so many struct page pointers, we should be passing around either
> > folio + offset, or we should be passing around struct f2fs_inode pointers.
> > My preference is for the latter.  We can always convert back to the
> > folio containing the inode if we need to (eg to mark it dirty) and it
> > adds some typesafety by ensuring that we're passing around pointers that
> > we believe belong to an inode and not, say, a struct page which happens
> > to contain a directory entry.
> > 
> > This is a monster task, I think.  I'm going to have to disable f2fs
> > from testing with split page/folio.  This is going to be a big problem
> > for Android.
> 
> I see. fyi; in Android, I'm thinking to run 16KB page kernel with 16KB format
> natively to keep block_size = PAGE_SIZE. Wasn't large folio to support a set
> of pages while keeping block_size = PAGE_SIZE?

Oh, I think I do see a possible argument for continuing this work.

If we have an f2fs filesystem with a 16kB block size, we can use order-0
folios with a 16kB PAGE_SIZE kernel, and if we want to mount it on a
kernel with a 4kB PAGE_SIZE kernel, then we can use order-2 folios to
do that.

Is that a useful improvement to f2fs?  It's not really the intent of
large folios; it's supposed to be used to support arbitrary order folios.
But we have all the pieces in place such that we could tell the page
cache min-order = max-order = 2.