Re: [PATCH 1/3] zram: fix operator precedence to get offset

Minchan Kim
2017-04-17T23:53:20Z

Hi Sergey,

On Mon, Apr 17, 2017 at 07:50:16PM +0900, Sergey Senozhatsky wrote:
> Hello Minchan,
>
> On (04/17/17 11:14), Minchan Kim wrote:
> > On Mon, Apr 17, 2017 at 10:54:29AM +0900, Sergey Senozhatsky wrote:
> > > On (04/17/17 10:21), Sergey Senozhatsky wrote:
> > > > > However, it should be *fixed* to prevent confusion in future
> > >
> > > or may be something like below? can save us some cycles.
> > >
> > > remove this calculation
> > >
> > > - offset = sector & (SECTORS_PER_PAGE - 1) << SECTOR_SHIFT;
> > >
> > >
> > > and pass 0 to zram_bvec_rw()
> > >
> > > - err = zram_bvec_rw(zram, &bv, index, offset, is_write);
> > > + err = zram_bvec_rw(zram, &bv, index, 0, is_write);
> >
> > That was one I wrote but have thought it more.
> >
> > Because I suspect fs can submit page-size IO in non-aligned PAGE_SIZE
> > sector? For example, it can submit PAGE_SIZE read request from 9 sector.
> > Is it possible? I don't know.
> >
> > As well, FS can format zram from sector 1, not sector 0? IOW, can't it
> > use starting sector as non-page algined sector?
> > We can do it via fdisk?
> >
> > Anyway, If one of scenario I mentioned is possible, zram_rw_page will
> > be broken.
> >
> > If it's hard to check all of scenario in this moment, it would be
> > better to not remove it and then add WARN_ON(offset) in there.
> >
> > While I am writing this, I found this.
> >
> > /**
> > * bdev_read_page() - Start reading a page from a block device
> > * @bdev: The device to read the page from
> > * @sector: The offset on the device to read the page to (need not be aligned)
> > * @page: The page to read
> > *
> >
> > Hmm,, need investigation but no time.
>
> good questions.
>
> as far as I can see, we never use 'offset' which we pass to zram_bvec_rw()
> from zram_rw_page(). `offset' makes a lot of sense for partial IO, but in
> zram_bvec_rw() we always do "bv.bv_len = PAGE_SIZE".
>
> so what we have is
>
> for READ
>
> zram_rw_page()
> bv.bv_len = PAGE_SIZE
> zram_bvec_rw(zram, &bv, index, offset, is_write);
> zram_bvec_read()
> if (is_partial_io(bvec)) // always false
> memcpy(user_mem + bvec->bv_offset,
> uncmem + offset,
> bvec->bv_len);
>
>
> for WRITE
>
> zram_rw_page()
> bv.bv_len = PAGE_SIZE
> zram_bvec_rw(zram, &bv, index, offset, is_write);
> zram_bvec_write()
> if (is_partial_io(bvec)) // always false
> memcpy(uncmem + offset,
> user_mem + bvec->bv_offset,
> bvec->bv_len);
>
>
> and our is_partial_io() looks at ->bv_len:
>
> bvec->bv_len != PAGE_SIZE;
>
> which we set to PAGE_SIZE.
>
> so in the existing scheme of things, we never care about 'sector'
> passed from zram_rw_page(). and this has worked for us for quite
> some time. my call would be -- let's drop zram_rw_page() `sector'
> calculation.

I can do but before that, I want to confirm. Ccing Matthew,
Summary for Matthew,

I see following comment about the sector from bdev_read_page.

/**
* bdev_read_page() - Start reading a page from a block device
* @bdev: The device to read the page from
* @sector: The offset on the device to read the page to (need not be aligned)
* @page: The page to read
*

Does it mean that sector can be not aligned PAGE_SIZE?

For example, 512byte sector, 4K page system, 4K = 8 sector

bdev_read_page(bdev, 9, page);

is possible for driver declared below?

blk_queue_physical_block_size(zram->disk->queue, PAGE_SIZE);
blk_queue_logical_block_size(zram->disk->queue,
ZRAM_LOGICAL_BLOCK_SIZE);

ZRAM_LOGICAL_BLOCK_SIZE is 4K regradless of 4K/64K page architecure.