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

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Message-ID: <20170417105016.GF518@jagdpanzerIV.localdomain>
Date:   Mon, 17 Apr 2017 19:50:16 +0900
From:   Sergey Senozhatsky <sergey.senozhatsky.work@...il.com>
To:     Minchan Kim <minchan@...nel.org>
Cc:     Sergey Senozhatsky <sergey.senozhatsky.work@...il.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        linux-kernel@...r.kernel.org, kernel-team@....com,
        stable@...r.kernel.org,
        Sergey Senozhatsky <sergey.senozhatsky@...il.com>
Subject: Re: [PATCH 1/3] zram: fix operator precedence to get offset

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