[PATCH v2 1/1] block: blk-merge: don't merge the pages with non-contiguous descriptors

On Thu, Jan 17, 2013 at 11:19:21AM +0000, James Bottomley wrote:

On Thu, 2013-01-17 at 11:04 +0000, Russell King - ARM Linux wrote:

quoted

On Thu, Jan 17, 2013 at 11:01:47AM +0000, James Bottomley wrote:

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On Thu, 2013-01-17 at 10:47 +0000, Russell King - ARM Linux wrote:

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Also, couldn't the addition of the scatterlist offset to the page also
be buggy too?

No, fortunately, offset must be within the first page from the point of
view of block generated sg lists.  As long as nothing within arm
violates this, it should be a safe assumption ... although the code
seems to assume otherwise.

Are you absolutely sure about that?  I believe I have seen cases where
that has been violated in the past, though it was many years ago.

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From the point of view of the block layer, absolutely: the scatterlist

is generated from an array of bio_vecs.  Each bio_vec is a page, offset
and length element and obeys the rule that offset must be within the
page and offset + length cannot stray over the page.

Well, I found it when working on the mmc stuff initially, long before
it got complex.  The scatterlists were unmodified from the block layer,
and I'm positive I saw occasions where the offset in the scatter lists
were larger than PAGE_SIZE.

quoted

From the point of view of other arm stuff, I don't know.

I'm not talking about anything ARM specific here.