On Fri, Feb 14, 2020 at 11:42:32AM -0800, Salman Qazi wrote: > On Fri, Feb 14, 2020 at 1:23 AM Ming Lei <tom.leiming@xxxxxxxxx> wrote: > > > > On Fri, Feb 14, 2020 at 1:50 PM Bart Van Assche <bvanassche@xxxxxxx> wrote: > > > > > > On 2020-02-13 11:21, Salman Qazi wrote: > > > > AFAICT, This is not actually sufficient, because the issuer of the bio > > > > is waiting for the entire bio, regardless of how it is split later. > > > > But, also there isn't a good mapping between the size of the secure > > > > discard and how long it will take. If given the geometry of a flash > > > > device, it is not hard to construct a scenario where a relatively > > > > small secure discard (few thousand sectors) will take a very long time > > > > (multiple seconds). > > > > > > > > Having said that, I don't like neutering the hung task timer either. > > > > > > Hi Salman, > > > > > > How about modifying the block layer such that completions of bio > > > fragments are considered as task activity? I think that bio splitting is > > > rare enough for such a change not to affect performance of the hot path. > > > > Are you sure that the task hung warning won't be triggered in case of > > non-splitting? > > I demonstrated a few emails ago that it doesn't take a very large > secure discard command to trigger this. So, I am sceptical that we > will be able to use splitting to solve this. > > > > > > > > > How about setting max_discard_segments such that a discard always > > > completes in less than half the hung task timeout? This may make > > > discards a bit slower for one particular block driver but I think that's > > > better than hung task complaints. > > > > I am afraid you can't find a golden setting max_discard_segments working > > for every drivers. Even it is found, the performance may have been affected. > > > > So just wondering why not take the simple approach used in blk_execute_rq()? > > My colleague Gwendal pointed out another issue which I had missed: > secure discard is an exclusive command: it monopolizes the device. > Even if we fix this via your approach, it will show up somewhere else, > because other operations to the drive will not make progress for that > length of time. What are the 'other operations'? Are they block IOs? If yes, that is why I suggest to fix submit_bio_wait(), which should cover most of sync bio submission. Anyway, the fix is simple & generic enough, I'd plan to post a formal patch if no one figures out better doable approaches. > > For Chromium OS purposes, if we had a blank slate, this is how I would solve it: > > * Under the assumption that the truly sensitive data is not very big: > * Keep secure data on a separate partition to make sure that those > LBAs have controlled history > * Treat the files in that partition as immutable (i.e. no > overwriting the contents of the file without first secure erasing the > existing contents). > * By never letting more than one version of the file accumulate, > we can guarantee that the secure erase will always be fast for > moderate sized files. > > But for all the existing machines with keys on them, we will need to > do something else. The issue you reported is a generic one, not Chromium only. Thanks, Ming
