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

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

On (04/18/17 08:53), Minchan Kim wrote:
> 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);

do you mean a sector that spans two pages? sectors are pow of 2 in size
and pages are pow of 2 in size, so page_size is `K * sector_size', isn't
it?

fs/mpage.c

static struct bio *
do_mpage_readpage(struct bio *bio, struct page *page, unsigned nr_pages,
                sector_t *last_block_in_bio, struct buffer_head *map_bh,
                unsigned long *first_logical_block, get_block_t get_block,
                gfp_t gfp)
{
        const unsigned blkbits = inode->i_blkbits;
        const unsigned blocks_per_page = PAGE_SIZE >> blkbits;
        const unsigned blocksize = 1 << blkbits;
        sector_t block_in_file;
        sector_t last_block;
        sector_t last_block_in_file;
        sector_t blocks[MAX_BUF_PER_PAGE];
	...
        block_in_file = (sector_t)page->index << (PAGE_SHIFT - blkbits);
        last_block = block_in_file + nr_pages * blocks_per_page;
        last_block_in_file = (i_size_read(inode) + blocksize - 1) >> blkbits;
        if (last_block > last_block_in_file)
                last_block = last_block_in_file;

or did I misunderstood your question?

	-ss