On Wed, Feb 06, 2019 at 07:16:21PM +0000, Christopher Lameter wrote: > On Wed, 6 Feb 2019, Doug Ledford wrote: > > > Most of the cases we want revoke for are things like truncate(). > > > Shouldn't happen with a sane system, but we're trying to avoid users > > > doing awful things like being able to DMA to pages that are now part of > > > a different file. > > > > Why is the solution revoke then? Is there something besides truncate > > that we have to worry about? I ask because EBUSY is not currently > > listed as a return value of truncate, so extending the API to include > > EBUSY to mean "this file has pinned pages that can not be freed" is not > > (or should not be) totally out of the question. > > > > Admittedly, I'm coming in late to this conversation, but did I miss the > > portion where that alternative was ruled out? > > Coming in late here too but isnt the only DAX case that we are concerned > about where there was an mmap with the O_DAX option to do direct write There is no O_DAX option. There's mount -o dax, but there's nothing that a program does to say "Use DAX". > though? If we only allow this use case then we may not have to worry about > long term GUP because DAX mapped files will stay in the physical location > regardless. ... except for truncate. And now that I think about it, there was a desire to support hot-unplug which also needed revoke. > Maybe we can solve the long term GUP problem through the requirement that > user space acquires some sort of means to pin the pages? In the DAX case > this is given by the filesystem and the hardware will basically take care > of writeback. It's not given by the filesystem. > In case of anonymous memory this can be guaranteed otherwise and is less > critical since these pages are not part of the pagecache and are not > subject to writeback. but are subject to being swapped out?
