linux-kernel - Re: [PATCH 04/11] block: Add bio_for_each_thp_segment

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Date:   Tue, 1 Sep 2020 15:50:25 +0100
From:   Christoph Hellwig <hch@...radead.org>
To:     Matthew Wilcox <willy@...radead.org>
Cc:     Christoph Hellwig <hch@...radead.org>, linux-xfs@...r.kernel.org,
        linux-fsdevel@...r.kernel.org,
        "Darrick J . Wong" <darrick.wong@...cle.com>,
        linux-block@...r.kernel.org, linux-mm@...ck.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH 04/11] block: Add bio_for_each_thp_segment_all

On Tue, Sep 01, 2020 at 02:05:25PM +0100, Matthew Wilcox wrote:
> > >                 struct page *page = bvec->bv_page;
> > > 
> > >                 while (length > 0) { 
> > >                         size_t count = thp_size(page) - offset;
> > >                         
> > >                         if (count > length)
> > >                                 count = length;
> > >                         iomap_read_page_end_io(page, offset, count, error);
> > >                         page += (offset + count) / PAGE_SIZE;
> > 
> > Shouldn't the page_size here be thp_size?
> 
> No.  Let's suppose we have a 20kB I/O which starts on a page boundary and
> the first page is order-2.  To get from the first head page to the second
> page, we need to add 4, which is 16kB / 4kB, not 16kB / 16kB.

True.

> I'm not entirely sure the bvec would shrink.  On 64-bit systems, it's
> currently 8 bytes for the struct page, 4 bytes for the len and 4 bytes
> for the offset.  Sure, we can get rid of the offset, but the compiler
> will just pad the struct from 12 bytes back to 16.  On 32-bit systems
> with 32-bit phys_addr_t, we go from 12 bytes down to 8, but most 32-bit
> systems have a 64-bit phys_addr_t these days, don't they?

Actually on those system that still are 32-bit because they are so
tiny I'd very much still expect a 32-bit phys_addr_t.  E.g. arm
without LPAE or 32-bit RISC-V.

But yeah, point taken on the alignment for the 64-bit ones.

> That's a bit more boilerplate than I'd like, but if bio_vec is going to
> lose its bv_page then I don't see a better way.  Unless we come up with
> a different page/offset/length struct that bio_vecs are decomposed into.

I'm not sure it is going to lose bv_page any time soon.  I'd sure like
to, but least time something like that came up Linus wasn't entirely
in favor.  Things might have changed now, though and I think it is about
time to give it another try.