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

Sergey Senozhatsky
2017-04-17T10:50:17Z

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.

-ss