From: David Sterba <dsterba@suse.cz>
To: "Gustavo A. R. Silva" <gustavoars@kernel.org>
Cc: Qu Wenruo <quwenruo.btrfs@gmx.com>,
"Gustavo A. R. Silva" <gustavo@embeddedor.com>,
David Sterba <dsterba@suse.com>,
linux-btrfs@vger.kernel.org
Subject: Re: [PATCH] btrfs: add special case to setget helpers for 64k pages
Date: Fri, 2 Jul 2021 12:22:42 +0200 [thread overview]
Message-ID: <20210702102242.GD2610@twin.jikos.cz> (raw)
In-Reply-To: <20210702003936.GA13456@embeddedor>
On Thu, Jul 01, 2021 at 07:39:36PM -0500, Gustavo A. R. Silva wrote:
> On Fri, Jul 02, 2021 at 08:21:31AM +0800, Qu Wenruo wrote:
> > On 2021/7/2 上午8:09, Gustavo A. R. Silva wrote:
> > > On 7/1/21 18:59, Qu Wenruo wrote:
> > > > On 2021/7/2 上午5:57, Gustavo A. R. Silva wrote:
> > > > > On Thu, Jul 01, 2021 at 06:00:39PM +0200, David Sterba wrote:
> > > > > > On 64K pages the size of the extent_buffer::pages array is 1 and
> > > > > > compilation with -Warray-bounds warns due to
> > > > > >
> > > > > > kaddr = page_address(eb->pages[idx + 1]);
> > > > > >
> > > > > > when reading byte range crossing page boundary.
> > > > > >
> > > > > > This does never actually overflow the array because on 64K because all
> > > > > > the data fit in one page and bounds are checked by check_setget_bounds.
> > > > > >
> > > > > > To fix the reported overflow and warning add a copy of the non-crossing
> > > > > > read/write code and put it behind a condition that's evaluated at
> > > > > > compile time. That way only one implementation remains due to dead code
> > > > > > elimination.
> > > > >
> > > > > Any chance we can use a flexible-array in struct extent_buffer instead,
> > > > > so all the warnings are removed?
> > > > >
> > > > > Something like this:
> > > > >
> > > > > diff --git a/fs/btrfs/extent_io.h b/fs/btrfs/extent_io.h
> > > > > index 62027f551b44..b82e8b694a3b 100644
> > > > > --- a/fs/btrfs/extent_io.h
> > > > > +++ b/fs/btrfs/extent_io.h
> > > > > @@ -94,11 +94,11 @@ struct extent_buffer {
> > > > >
> > > > > struct rw_semaphore lock;
> > > > >
> > > > > - struct page *pages[INLINE_EXTENT_BUFFER_PAGES];
> > > > > struct list_head release_list;
> > > > > #ifdef CONFIG_BTRFS_DEBUG
> > > > > struct list_head leak_list;
> > > > > #endif
> > > > > + struct page *pages[];
> > > > > };
> > > >
> > > > But wouldn't that make the the size of extent_buffer structure change
> > > > and affect the kmem cache for it?
> > >
> > > Could you please point out the places in the code that would be
> > > affected?
> >
> > Sure, the direct code get affected is here:
> >
> > extent_io.c:
> > int __init extent_io_init(void)
> > {
> > extent_buffer_cache = kmem_cache_create("btrfs_extent_buffer",
> > sizeof(struct extent_buffer), 0,
> > SLAB_MEM_SPREAD, NULL);
> >
> > So here we can no longer use sizeof(struct extent_buffer);
> >
> > Furthermore, this function is called at btrfs module load time,
> > at that time we have no idea how large the extent buffer could be, thus we
> > must allocate a large enough cache for extent buffer.
> >
> > Thus the size will be fixed to the current size, no matter if we use flex
> > array or not.
> >
> > Though I'm not sure if using such flex array with fixed real size can silent
> > the warning though.
>
> Yeah; I think this might be the right solution:
>
> diff --git a/fs/btrfs/extent_io.c b/fs/btrfs/extent_io.c
> index 9e81d25dea70..4cf0b72fdd9f 100644
> --- a/fs/btrfs/extent_io.c
> +++ b/fs/btrfs/extent_io.c
> @@ -232,8 +232,9 @@ int __init extent_state_cache_init(void)
> int __init extent_io_init(void)
> {
> extent_buffer_cache = kmem_cache_create("btrfs_extent_buffer",
> - sizeof(struct extent_buffer), 0,
> - SLAB_MEM_SPREAD, NULL);
> + struct_size((struct extent_buffer *)0, pages,
> + INLINE_EXTENT_BUFFER_PAGES),
> + 0, SLAB_MEM_SPREAD, NULL);
> if (!extent_buffer_cache)
> return -ENOMEM;
>
> diff --git a/fs/btrfs/extent_io.h b/fs/btrfs/extent_io.h
> index 62027f551b44..b82e8b694a3b 100644
> --- a/fs/btrfs/extent_io.h
> +++ b/fs/btrfs/extent_io.h
> @@ -94,11 +94,11 @@ struct extent_buffer {
>
> struct rw_semaphore lock;
>
> - struct page *pages[INLINE_EXTENT_BUFFER_PAGES];
> struct list_head release_list;
> #ifdef CONFIG_BTRFS_DEBUG
> struct list_head leak_list;
> #endif
> + struct page *pages[];
IMHO this is going the wrong way, INLINE_EXTENT_BUFFER_PAGES is a
compile time constant and the array is not variable sized at all, so
adding the end member and using struct_size is just manually coding
what would compiler do for free.
next prev parent reply other threads:[~2021-07-02 10:25 UTC|newest]
Thread overview: 16+ messages / expand[flat|nested] mbox.gz Atom feed top
2021-07-01 16:00 [PATCH] btrfs: add special case to setget helpers for 64k pages David Sterba
2021-07-01 21:57 ` Gustavo A. R. Silva
2021-07-01 23:59 ` Qu Wenruo
2021-07-02 0:09 ` Gustavo A. R. Silva
2021-07-02 0:21 ` Qu Wenruo
2021-07-02 0:39 ` Gustavo A. R. Silva
2021-07-02 0:39 ` Qu Wenruo
2021-07-02 1:09 ` Gustavo A. R. Silva
2021-07-02 10:22 ` David Sterba [this message]
2021-07-02 7:10 ` Christoph Hellwig
2021-07-02 11:06 ` David Sterba
2021-07-05 8:33 ` Christoph Hellwig
2021-07-08 14:34 ` David Sterba
2021-07-14 23:37 ` Gustavo A. R. Silva
2021-07-28 15:32 ` David Sterba
2021-07-28 16:00 ` David Sterba
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