Hi Hugh, On Fri, Mar 29, 2013 at 01:01:14PM -0700, Hugh Dickins wrote: > On Fri, 29 Mar 2013, Minchan Kim wrote: > > On Thu, Mar 28, 2013 at 11:19:12AM -0700, Dan Magenheimer wrote: > > > > From: Minchan Kim [mailto:minchan@xxxxxxxxxx] > > > > On Wed, Mar 27, 2013 at 03:24:00PM -0700, Dan Magenheimer wrote: > > > > > > From: Hugh Dickins [mailto:hughd@xxxxxxxxxx] > > > > > > Subject: Re: [RFC] mm: remove swapcache page early > > > > > > > > > > > > I believe the answer is for frontswap/zmem to invalidate the frontswap > > > > > > copy of the page (to free up the compressed memory when possible) and > > > > > > SetPageDirty on the PageUptodate PageSwapCache page when swapping in > > > > > > (setting page dirty so nothing will later go to read it from the > > > > > > unfreed location on backing swap disk, which was never written). > > > > > > > > > > There are two duplication issues: (1) When can the page be removed > > > > > from the swap cache after a call to frontswap_store; and (2) When > > > > > can the page be removed from the frontswap storage after it > > > > > has been brought back into memory via frontswap_load. > > > > > > > > > > This patch from Minchan addresses (1). The issue you are raising > > > > > > > > No. I am addressing (2). > > > > > > > > > here is (2). You may not know that (2) has recently been solved > > > > > in frontswap, at least for zcache. See frontswap_exclusive_gets_enabled. > > > > > If this is enabled (and it is for zcache but not yet for zswap), > > > > > what you suggest (SetPageDirty) is what happens. > > > > > > > > I am blind on zcache so I didn't see it. Anyway, I'd like to address it > > > > on zram and zswap. > > > > > > Zswap can enable it trivially by adding a function call in init_zswap. > > > (Note that it is not enabled by default for all frontswap backends > > > because it is another complicated tradeoff of cpu time vs memory space > > > that needs more study on a broad set of workloads.) > > > > > > I wonder if something like this would have a similar result for zram? > > > (Completely untested... snippet stolen from swap_entry_free with > > > SetPageDirty added... doesn't compile yet, but should give you the idea.) > > Thanks for correcting me on zram (in earlier mail of this thread), yes, > I was forgetting about the swap_slot_free_notify entry point which lets > that memory be freed. > > > > > Nice idea! > > > > After I see your patch, I realized it was Hugh's suggestion and > > you implemented it in proper place. > > > > Will resend it after testing. Maybe nextweek. > > Thanks! > > Be careful, although Dan is right that something like this can be > done for zram, I believe you will find that it needs a little more: > either a separate new entry point (not my preference) or a flags arg > (or boolean) added to swap_slot_free_notify. > > Because this is a different operation: end_swap_bio_read() wants > to free up zram's compressed copy of the page, but the swp_entry_t > must remain valid until swap_entry_free() can clear up the rest. > Precisely how much of the work each should do, you will discover. First of all, Thanks for noticing it for me! If I parse your concern correctly, you are concerning about different semantic on two functions. (end_swap_bio_read's swap_slot_free_notify VS swap_entry_free's one). But current implementatoin on zram_slot_free_notify could cover both cases properly with luck. zram_free_page caused by end_swap_bio_read will free compressed copy of the page and zram_free_page caused by swap_entry_free later won't find right index from zram->table and just return. So I think there is no problem. Remained problem is zram->stats.notify_free, which could be counted redundantly but not sure it's valuable to count exactly. If I miss your point, please pinpoint your concern. :) Thanks! -- Kind regards, Minchan Kim -- To unsubscribe, send a message with 'unsubscribe linux-mm' in the body to majordomo@xxxxxxxxx. For more info on Linux MM, see: http://www.linux-mm.org/ . Don't email: <a href=mailto:"dont@xxxxxxxxx";> email@xxxxxxxxx </a>