Re: [PATCH] nfs/blocklayout: make sure making a aligned read request

[Kinglong Mee <kinglongmee@gmail.com>]

On 2/29/2016 17:57, Christoph Hellwig wrote:
> On Sat, Feb 13, 2016 at 09:51:31PM +0800, Kinglong Mee wrote:
>> Only treat write goes up to the inode size as aligned request,
>> because it always write PAGE_CACHE_SIZE, but read a dynamic size.
> 
> Can you explain what the point is? 

I run ltp tests with read02 hang.
There seams a loop in block codes.
It is caused by passing an unaligned read to bio.
So this patch is out as making a aligned read request.

> We'll never use data pas the block size
> in the page cache, but per the block size requirement in the spec we must
> be able to read it.  This patch means we can't direct storage reads where
> we previously could, without any obvious upside.

bl_pg_init_read/bl_pg_test_read checks aligned base on SECTOR_SIZE.
bl_pg_init_write/bl_pg_test_write checks aligned base on PAGE_SIZE.

If according the codes, reads data per block size is okay.

But, there is a comment in bl_read_pagelist() as,

250         isect = (sector_t) (f_offset >> SECTOR_SHIFT);
251         /* Code assumes extents are page-aligned */
252         for (i = pg_index; i < header->page_array.npages; i++) {
253                 if (extent_length <= 0) {

I don't known the meaning of "extents are page-aligned", 
extent's start offset is aligned to page size?
or extent's start offset is aligned to page size and length
is equal to PAGE_SIZE too ?

thanks,
Kinglong Mee