Messages in this thread | | | Date | Tue, 09 Apr 2013 13:36:52 +0800 | From | Ric Mason <> | Subject | Re: [PATCH] mm: remove compressed copy from zram in-memory |
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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. > > >> -- >> To unsubscribe, send a message with 'unsubscribe linux-mm' in >> the body to majordomo@kvack.org. For more info on Linux MM, >> see: http://www.linux-mm.org/ . >> Don't email: <a href=mailto:"dont@kvack.org"> email@kvack.org </a>
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