On Fri, Oct 22, 2021 at 01:37:28AM +0000, Jane Chu wrote: > On 10/21/2021 4:31 AM, Christoph Hellwig wrote: > > Looking over the series I have serious doubts that overloading the > > slow path clear poison operation over the fast path read/write > > path is such a great idea. Why would data recovery after a media error ever be considered a fast/hot path? A normal read/write to a fsdax file would not pass the flag, which skips the poison checking with whatever MCE consequences that has, right? pwritev2(..., RWF_RECOVER_DATA) should be infrequent enough that carefully stepping around dax_direct_access only has to be faster than restoring from backup, I hope? > Understood, sounds like a concern on principle. But it seems to me > that the task of recovery overlaps with the normal write operation > on the write part. Without overloading some write operation for > 'recovery', I guess we'll need to come up with a new userland > command coupled with a new dax API ->clear_poison and propagate the > new API support to each dm targets that support dax which, again, > is an idea that sounds too bulky if I recall Dan's earlier rejection > correctly. > > It is in my plan though, to provide pwritev2(2) and preadv2(2) man pages > with description about the RWF_RECOVERY_DATA flag and specifically not > recommending the flag for normal read/write operation - due to potential > performance impact from searching badblocks in the range. Yes, this will help much. :) > That said, the badblock searching is turned on only if the pmem device > contains badblocks(i.e. bb->count > 0), otherwise, the performance > impact is next to none. And once the badblock search starts, > it is a binary search over user provided range. The unwanted impact > happens to be the case when the pmem device contains badblocks > that do not fall in the user specified range, and in that case, the > search would end in O(1). (I wonder about improving badblocks to be less sector-oriented and not have that weird 16-records-max limit, but that can be a later optimization.) --D > Thanks! > -jane > >
