Re: [PATCH] mm: remove compressed copy from zram in-memory

[Ric Mason <ric.masonn@gmail.com>]

Hi Minchan,
On 04/09/2013 09:02 AM, Minchan Kim wrote:
> Hi Andrew,
>
> On Mon, Apr 08, 2013 at 02:17:10PM -0700, Andrew Morton wrote:
>> On Mon,  8 Apr 2013 15:01:02 +0900 Minchan Kim <minchan@kernel.org> wrote:
>>
>>> Swap subsystem does lazy swap slot free with expecting the page
>>> would be swapped out again so we can avoid unnecessary write.
>> Is that correct?  How can it save a write?
> Correct.
>
> The add_to_swap makes the page dirty and we must pageout only if the page is
> dirty. If a anon page is already charged into swapcache, we skip writeout
> the page in shrink_page_list, then just remove the page from swapcache and
> free it by __remove_mapping.
>
> I did received same question multiple time so it would be good idea to
> write down it in vmscan.c somewhere.
>
>>> But the problem in in-memory swap(ex, zram) is that it consumes
>>> memory space until vm_swap_full(ie, used half of all of swap device)
>>> condition meet. It could be bad if we use multiple swap device,
>>> small in-memory swap and big storage swap or in-memory swap alone.
>>>
>>> This patch makes swap subsystem free swap slot as soon as swap-read
>>> is completed and make the swapcache page dirty so the page should
>>> be written out the swap device to reclaim it.
>>> It means we never lose it.
>> >From my reading of the patch, that isn't how it works?  It changed
>> end_swap_bio_read() to call zram_slot_free_notify(), which appears to
>> free the underlying compressed page.  I have a feeling I'm hopelessly
>> confused.
> You understand right totally.
> Selecting swap slot in my description was totally miss.
> Need to rewrite the description.

free the swap slot and free compress page is the same, isn't it?

>
>>> --- a/mm/page_io.c
>>> +++ b/mm/page_io.c
>>> @@ -20,6 +20,7 @@
>>>   #include <linux/buffer_head.h>
>>>   #include <linux/writeback.h>
>>>   #include <linux/frontswap.h>
>>> +#include <linux/blkdev.h>
>>>   #include <asm/pgtable.h>
>>>   
>>>   static struct bio *get_swap_bio(gfp_t gfp_flags,
>>> @@ -81,8 +82,30 @@ void end_swap_bio_read(struct bio *bio, int err)
>>>   				iminor(bio->bi_bdev->bd_inode),
>>>   				(unsigned long long)bio->bi_sector);
>>>   	} else {
>>> +		/*
>>> +		 * There is no reason to keep both uncompressed data and
>>> +		 * compressed data in memory.
>>> +		 */
>>> +		struct swap_info_struct *sis;
>>> +
>>>   		SetPageUptodate(page);
>>> +		sis = page_swap_info(page);
>>> +		if (sis->flags & SWP_BLKDEV) {
>>> +			struct gendisk *disk = sis->bdev->bd_disk;
>>> +			if (disk->fops->swap_slot_free_notify) {
>>> +				swp_entry_t entry;
>>> +				unsigned long offset;
>>> +
>>> +				entry.val = page_private(page);
>>> +				offset = swp_offset(entry);
>>> +
>>> +				SetPageDirty(page);
>>> +				disk->fops->swap_slot_free_notify(sis->bdev,
>>> +						offset);
>>> +			}
>>> +		}
>>>   	}
>>> +
>>>   	unlock_page(page);
>>>   	bio_put(bio);
>> The new code is wasted space if CONFIG_BLOCK=n, yes?
> CONFIG_SWAP is already dependent on CONFIG_BLOCK.
>
>> Also, what's up with the SWP_BLKDEV test?  zram doesn't support
>> SWP_FILE?  Why on earth not?
>>
>> Putting swap_slot_free_notify() into block_device_operations seems
>> rather wrong.  It precludes zram-over-swapfiles for all time and means
>> that other subsystems cannot get notifications for swap slot freeing
>> for swapfile-backed swap.
> Zram is just pseudo-block device so anyone can format it with any FSes
> and swapon a file. In such case, he can't get a benefit from
> swap_slot_free_notify. But I think it's not a severe problem because
> there is no reason to use a file-swap on zram. If anyone want to use it,
> I'd like to know the reason. If it's reasonable, we have to rethink a
> wheel and it's another story, IMHO.
>
>
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