On Mon, Aug 5, 2019 at 12:17 PM Boaz Harrosh <boaz@xxxxxxxxxxxxx> wrote: > > On 05/08/2019 21:49, Vivek Goyal wrote: > > On Mon, Aug 05, 2019 at 02:53:06PM +0300, Boaz Harrosh wrote: > <> > >> So as I understand the man page: > >> fallocate(FL_PUNCH_HOLE); means user is asking to get rid also of COW pages. > >> On the other way fallocate(FL_ZERO_RANGE) only the pmem portion is zeroed and COW (private pages) stays > > > > I tested fallocate(FL_PUNCH_HOLE) on xfs (non-dax) and it does not seem to > > get rid of COW pages and my test case still can read the data it wrote > > in private pages. > > > > It seems you are right and I am wrong. This is what the Kernel code has to say about it: > > /* > * Unlike in truncate_pagecache, unmap_mapping_range is called only > * once (before truncating pagecache), and without "even_cows" flag: > * hole-punching should not remove private COWed pages from the hole. > */ > > For me this is confusing but that is what it is. So remove private COWed pages > is only done when we do an setattr(ATTR_SIZE). > > >> > >> Just saying I have not followed the above code path > >> (We should have an xfstest for this?) > > > > I don't know either. It indeed is interesting to figure out what's the > > expected behavior with fallocate() and truncate() for COW pages and cover > > that using xfstest (if not already done). > > > > I could not find any test for the COW positive FL_PUNCH_HOLE (I have that bug) > could be nice to make one, and let FSs like mine fail. > Any way very nice catch. > Yes, and this bug is worse because it affects COW pages that are not the direct target of the truncate / hole punch. This unmap in dax_layout_busy_page() is only there to allow the fs to synchronize against get_user_pages_fast() which might otherwise race to grab a page reference and prevent the fs from making forward progress. The unmap_mapping_range() that addresses COW pages in the truncated range occurs later after the filesystem has regained control of the extent layout (i.e. break layouts has succeeded).
