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

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Date:   Tue, 18 Apr 2017 08:53:10 +0900
From:   Minchan Kim <minchan@...nel.org>
To:     Sergey Senozhatsky <sergey.senozhatsky.work@...il.com>,
        <willy@...radead.org>
CC:     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

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.