Dan Williams <dan.j.williams@xxxxxxxxx> writes: > On Sun, Feb 23, 2020 at 3:03 PM Dave Chinner <david@xxxxxxxxxxxxx> wrote: >> >> On Fri, Feb 21, 2020 at 03:17:59PM -0500, Vivek Goyal wrote: >> > On Fri, Feb 21, 2020 at 01:32:48PM -0500, Jeff Moyer wrote: >> > > Vivek Goyal <vgoyal@xxxxxxxxxx> writes: >> > > >> > > > On Thu, Feb 20, 2020 at 04:35:17PM -0500, Jeff Moyer wrote: >> > > >> Vivek Goyal <vgoyal@xxxxxxxxxx> writes: >> > > >> >> > > >> > Currently pmem_clear_poison() expects offset and len to be sector aligned. >> > > >> > Atleast that seems to be the assumption with which code has been written. >> > > >> > It is called only from pmem_do_bvec() which is called only from pmem_rw_page() >> > > >> > and pmem_make_request() which will only passe sector aligned offset and len. >> > > >> > >> > > >> > Soon we want use this function from dax_zero_page_range() code path which >> > > >> > can try to zero arbitrary range of memory with-in a page. So update this >> > > >> > function to assume that offset and length can be arbitrary and do the >> > > >> > necessary alignments as needed. >> > > >> >> > > >> What caller will try to zero a range that is smaller than a sector? >> > > > >> > > > Hi Jeff, >> > > > >> > > > New dax zeroing interface (dax_zero_page_range()) can technically pass >> > > > a range which is less than a sector. Or which is bigger than a sector >> > > > but start and end are not aligned on sector boundaries. >> > > >> > > Sure, but who will call it with misaligned ranges? >> > >> > create a file foo.txt of size 4K and then truncate it. >> > >> > "truncate -s 23 foo.txt". Filesystems try to zero the bytes from 24 to >> > 4095. >> >> This should fail with EIO. Only full page writes should clear the >> bad page state, and partial writes should therefore fail because >> they do not guarantee the data in the filesystem block is all good. >> >> If this zeroing was a buffered write to an address with a bad >> sector, then the writeback will fail and the user will (eventually) >> get an EIO on the file. >> >> DAX should do the same thing, except because the zeroing is >> synchronous (i.e. done directly by the truncate syscall) we can - >> and should - return EIO immediately. >> >> Indeed, with your code, if we then extend the file by truncating up >> back to 4k, then the range between 23 and 512 is still bad, even >> though we've successfully zeroed it and the user knows it. An >> attempt to read anywhere in this range (e.g. 10 bytes at offset 100) >> will fail with EIO, but reading 10 bytes at offset 2000 will >> succeed. >> >> That's *awful* behaviour to expose to userspace, especially when >> they look at the fs config and see that it's using both 4kB block >> and sector sizes... >> >> The only thing that makes sense from a filesystem perspective is >> clearing bad page state when entire filesystem blocks are >> overwritten. The data in a filesystem block is either good or bad, >> and it doesn't matter how many internal (kernel or device) sectors >> it has. >> >> > > And what happens to the rest? The caller is left to trip over the >> > > errors? That sounds pretty terrible. I really think there needs to be >> > > an explicit contract here. >> > >> > Ok, I think is is the contentious bit. Current interface >> > (__dax_zero_page_range()) either clears the poison (if I/O is aligned to >> > sector) or expects page to be free of poison. >> > >> > So in above example, of "truncate -s 23 foo.txt", currently I get an error >> > because range being zeroed is not sector aligned. So >> > __dax_zero_page_range() falls back to calling direct_access(). Which >> > fails because there are poisoned sectors in the page. >> > >> > With my patches, dax_zero_page_range(), clears the poison from sector 1 to >> > 7 but leaves sector 0 untouched and just writes zeroes from byte 0 to 511 >> > and returns success. >> >> Ok, kernel sectors are not the unit of granularity bad page state >> should be managed at. They don't match page state granularity, and >> they don't match filesystem block granularity, and the whacky >> "partial writes silently succeed, reads fail unpredictably" >> assymetry it leads to will just cause problems for users. >> >> > So question is, is this better behavior or worse behavior. If sector 0 >> > was poisoned, it will continue to remain poisoned and caller will come >> > to know about it on next read and then it should try to truncate file >> > to length 0 or unlink file or restore that file to get rid of poison. >> >> Worse, because the filesystem can't track what sub-parts of the >> block are bad and that leads to inconsistent data integrity status >> being exposed to userspace. > > The driver can't track it either. Latent poison isn't know until it is > consumed, and writes to latent poison are not guaranteed to clear it. I believe we're discussing the case where we know there is a bad block. Obviously we can't know about latent errors. >> > IOW, if a partial block is being zeroed and if it is poisoned, caller >> > will not be return an error and poison will not be cleared and memory >> > will be zeroed. What do we expect in such cases. >> > >> > Do we expect an interface where if there are any bad blocks in the range >> > being zeroed, then they all should be cleared (and hence all I/O should >> > be aligned) otherwise error is returned. If yes, I could make that >> > change. >> > >> > Downside of current interface is that it will clear as many blocks as >> > possible in the given range and leave starting and end blocks poisoned >> > (if it is unaligned) and not return error. That means a reader will >> > get error on these blocks again and they will have to try to clear it >> > again. >> >> Which is solved by having partial page writes always EIO on poisoned >> memory. > > The problem with the above is that partial page writes can not be > guaranteed to return EIO. Poison is only detected on consumed reads, > or a periodic scrub, not writes. IFF poison detection was always > synchronous with poison creation then the above makes sense. However, > with asynchronous signaling, it's fundamentally a false security > blanket to assume even full block writes will clear poison unless a > callback to firmware is made for every block. Let's just focus on reporting errors when we know we have them. -Jeff
