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From:   Vishal Moola <vishal.moola@gmail.com>
Date:   Wed, 16 Nov 2022 11:49:40 -0800
Message-ID: <CAOzc2pxAG2-y7eNTbhDzh6X_Aqzj4o7BVkzudmhdkTy6rghU7A@mail.gmail.com>
Subject: Re: [PATCH 1/4] ext4: Convert move_extent_per_page() to use folios
To:     Matthew Wilcox <willy@infradead.org>
Cc:     linux-mm@kvack.org, linux-fsdevel@vger.kernel.org,
        linux-kernel@vger.kernel.org, linux-ext4@vger.kernel.org,
        akpm@linux-foundation.org, naoya.horiguchi@nec.com, tytso@mit.edu
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On Wed, Nov 16, 2022 at 12:09 AM Matthew Wilcox <willy@infradead.org> wrote:
>
> On Tue, Nov 15, 2022 at 06:10:08PM -0800, Vishal Moola (Oracle) wrote:
> >  {
> >       struct inode *orig_inode = file_inode(o_filp);
> >       struct page *pagep[2] = {NULL, NULL};
> > +     struct folio *folio[2] = {NULL, NULL};
>
> I have a feeling that mext_page_double_lock() should also be converted
> to use folios.  But this makes me nervous:
>
>         int blocks_per_page = PAGE_SIZE >> orig_inode->i_blkbits;
>
> and I'm not sure what will happen if one or both of the orig_page
> and donor_page is large -- possibly different sizes of large.
>
> Obviously ext4 doesn't allow large folios today, but it would be good to
> get some reasoning about why this isn't laying a trap for later (or at
> least assertions that neither folio is large so that there's an obvious
> scream instead of silent data corruption).

I had thought once mext_page_mkuptodate() and block_commit_write()
were converted to folios, large folios wouldn't be a problem. I hadn't
considered
that the folios may be of different sizes. I can add assertions about both
folios being large and the same size in v2.