On 06/25/2018 08:21 AM, Jan Kara wrote: > On Thu 21-06-18 18:30:36, Jan Kara wrote: >> On Wed 20-06-18 15:55:41, John Hubbard wrote: >>> On 06/20/2018 05:08 AM, Jan Kara wrote: >>>> On Tue 19-06-18 11:11:48, John Hubbard wrote: >>>>> On 06/19/2018 03:41 AM, Jan Kara wrote: >>>>>> On Tue 19-06-18 02:02:55, Matthew Wilcox wrote: >>>>>>> On Tue, Jun 19, 2018 at 10:29:49AM +0200, Jan Kara wrote: >>> [...] > I've spent some time on this. There are two obstacles with my approach of > putting special entry into inode's VMA tree: > > 1) If I want to place this special entry in inode's VMA tree, I either need > to allocate full VMA, somehow initiate it so that it's clear it's a special > "pinned" range, not a VMA => uses unnecessarily too much memory, it is > ugly. Another solution I was hoping for was that I would factor out some > common bits of vm_area_struct (pgoff, rb_node, ..) into a structure common > for VMA and the locked range => doable but causes a lot of churn as VMAs > are accessed (and modified!) at hundreds of places in the kernel. Some > accessor functions would help to reduce the churn a bit but then stuff like > vma_set_pgoff(vma, pgoff) isn't exactly beautiful either. > > 2) Some users of GUP (e.g. direct IO) get a block of pages and then put > references to these pages at different times and in random order - > basically when IO for given page is completed, reference is dropped and one > GUP call can acquire page references for pages which end up in multiple > different bios (we don't know in advance). This makes is difficult to > implement counterpart to GUP to 'unpin' a range of pages - we'd either have > to support partial unpins (and splitting of pinned ranges and all such fun) > or just have to track internally in how many pages are still pinned in the > originally pinned range and release the pin once all individual pages are > unpinned but then it's difficult to e.g. get to this internal structure > from IO completion callback where we only have the bio. > > So I think the Matthew's idea of removing pinned pages from LRU is > definitely worth trying to see how complex that would end up being. Did you > get to looking into it? If not, I can probably find some time to try that > out. > OK. Even if we remove the pages from the LRU, we still have to insert a "put_gup_page" or similarly named call. But it could be a simple replacement for put_page, with that approach, so that does make it much much easier. I was (and still am) planning on tackling this today, so let me see how far I get before yelling for help. :) thanks, -- John Hubbard NVIDIA
