On Mon, Feb 24, 2020 at 12:13 PM Vivek Goyal <vgoyal@xxxxxxxxxx> wrote: > > On Mon, Feb 24, 2020 at 10:03:30AM +1100, Dave Chinner 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. > > > > > > > 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. > > Ok, how about if I add one more patch to the series which will check > if unwritten portion of the page has known poison. If it has, then > -EIO is returned. > > > Subject: pmem: zero page range return error if poisoned memory in unwritten area > > Filesystems call into pmem_dax_zero_page_range() to zero partial page upon > truncate. If partial page is being zeroed, then at the end of operation > file systems expect that there is no poison in the whole page (atleast > known poison). > > So make sure part of the partial page which is not being written, does not > have poison. If it does, return error. If there is poison in area of page > being written, it will be cleared. No, I don't like that the zero operation is special cased compared to the write case. I'd say let's make them identical for now. I.e. fail the I/O at dax_direct_access() time. I think the error clearing interface should be an explicit / separate op rather than a side-effect. What about an explicit interface for initializing newly allocated blocks, and the only reliable way to destroy poison through the filesystem is to free the block?
