Hi Matthew, On 2020-02-02 22:40:47 -0800, Matthew Wilcox wrote: > On Sat, Feb 01, 2020 at 01:43:09AM -0800, Andres Freund wrote: > > As far as I can tell POSIX_FADV_WILLNEED synchronously starts readahead, > > including page allocation etc, which of course might trigger quite > > blocking. The fs also quite possibly needs to read metadata. > > > > > > Seems like either WILLNEED would have to always be deferred, or > > force_page_cache_readahead, __do_page_cache_readahead would etc need to > > be wired up to know not to block. Including returning EAGAIN, despite > > force_page_cache_readahead and generic_readahead() intentially ignoring > > return values / errors. > > The first step is going to be letting the readahead code know that it > should have this behaviour, which is tricky because the code flow looks > like this: > > io_fadvise > vfs_fadvise > file->f_op->fadvise() Yea. > ... and we'd be breaking brand new ground trying to add a gfp_t to a > file_operations method. Which is not to say it couldn't be done, but > would mean changing filesystems, just so we could pass the gfp > flags through from the top level to the low level. It wouldn't be > too bad; only two filesystems implement an ->fadvise op today. I was wondering if the right approach could be to pass through a kiocb instead of gfp_t. There's obviously precedent for that in file_operations, and then IOCB_NOWAIT could be used to represent the the intent to not block. It'd be a bit weird in the sense that currently there'd probably be no callback - but that seems fairly minor. And who knows, > Next possibility, we could add a POSIX_FADV_WILLNEED_ASYNC advice > flag. POSIX_FADV_DONTNEED has similar problems to POSIX_FADV_WILLNEED, so it'd be nice to come up with an API change to vfs_fadvise that'd support both. Obviously there also could be a POSIX_FADV_DONTNEED_ASYNC, but ... > Something I already want to see in an entirely different context is > a flag in the task_struct which says, essentially, "don't block in > memory allocations" -- ie behave as if __GFP_NOWAIT | __GFP_NOWARN > is set. See my proposal here: I'm a bit out of my depth here: Would __GFP_NOWAIT actually be suitable to indicate that no blocking IO is to be executed by the FS? E.g. for metadata? As far as I can tell that's also a problem, not just reclaim to make space for the to-be-read data. > I've got my head stuck in the middle of the readahead code right now, > so this seems like a good time to add this functionality. Once I'm done > with finding out who broke my test VM, I'll take a shot at adding > this. Cool! Greetings, Andres Freund
