linux-kernel - Re: [PATCH 2/2] mm/filemap: fix mapping_seek_hole

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Date:   Thu, 22 Apr 2021 02:16:31 +0100
From:   Matthew Wilcox <willy@...radead.org>
To:     Hugh Dickins <hughd@...gle.com>
Cc:     Andrew Morton <akpm@...ux-foundation.org>,
        William Kucharski <william.kucharski@...cle.com>,
        Christoph Hellwig <hch@....de>, Jan Kara <jack@...e.cz>,
        Dave Chinner <dchinner@...hat.com>,
        Johannes Weiner <hannes@...xchg.org>,
        "Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com>,
        Yang Shi <yang.shi@...ux.alibaba.com>,
        linux-fsdevel@...r.kernel.org, linux-kernel@...r.kernel.org,
        linux-mm@...ck.org
Subject: Re: [PATCH 2/2] mm/filemap: fix mapping_seek_hole_data on THP &
 32-bit

On Wed, Apr 21, 2021 at 05:39:14PM -0700, Hugh Dickins wrote:
> No problem on 64-bit without huge pages, but xfstests generic/285
> and other SEEK_HOLE/SEEK_DATA tests have regressed on huge tmpfs,
> and on 32-bit architectures, with the new mapping_seek_hole_data().
> Several different bugs turned out to need fixing.
> 
> u64 casts added to stop unfortunate sign-extension when shifting
> (and let's use shifts throughout, rather than mixed with * and /).

That confuses me.  loff_t is a signed long long, but it can't be negative
(... right?)  So how does casting it to an u64 before dividing by
PAGE_SIZE help?

> Use round_up() when advancing pos, to stop assuming that pos was
> already THP-aligned when advancing it by THP-size.  (But I believe
> this use of round_up() assumes that any THP must be THP-aligned:
> true while tmpfs enforces that alignment, and is the only fs with
> FS_THP_SUPPORT; but might need to be generalized in the future?
> If I try to generalize it right now, I'm sure to get it wrong!)

No generalisation needed in future.  Folios must be naturally aligned
within a file.

> @@ -2681,7 +2681,8 @@ loff_t mapping_seek_hole_data(struct add
>  
>  	rcu_read_lock();
>  	while ((page = find_get_entry(&xas, max, XA_PRESENT))) {
> -		loff_t pos = xas.xa_index * PAGE_SIZE;
> +		loff_t pos = (u64)xas.xa_index << PAGE_SHIFT;
> +		unsigned int seek_size;

I've been preferring size_t for 'number of bytes in a page' because
I'm sure somebody is going to want a page larger than 2GB in the next
ten years.