On 09/07/2024 00:01, Dave Chinner wrote: > On Fri, Jul 05, 2024 at 02:31:08PM +0100, Ryan Roberts wrote: >> On 05/07/2024 14:24, Pankaj Raghav (Samsung) wrote: >>>>> I suggest you handle it better than this. If the device is asking for a >>>>> blocksize > PMD_SIZE, you should fail to mount it. >>>> >>>> That's my point: we already do that. >>>> >>>> The largest block size we support is 64kB and that's way smaller >>>> than PMD_SIZE on all platforms and we always check for bs > ps >>>> support at mount time when the filesystem bs > ps. >>>> >>>> Hence we're never going to set the min value to anything unsupported >>>> unless someone makes a massive programming mistake. At which point, >>>> we want a *hard, immediate fail* so the developer notices their >>>> mistake immediately. All filesystems and block devices need to >>>> behave this way so the limits should be encoded as asserts in the >>>> function to trigger such behaviour. >>> >>> I agree, this kind of bug will be encountered only during developement >>> and not during actual production due to the limit we have fs block size >>> in XFS. >>> >>>> >>>>> If the device is >>>>> asking for a blocksize > PAGE_SIZE and CONFIG_TRANSPARENT_HUGEPAGE is >>>>> not set, you should also decline to mount the filesystem. >>>> >>>> What does CONFIG_TRANSPARENT_HUGEPAGE have to do with filesystems >>>> being able to use large folios? >>>> >>>> If that's an actual dependency of using large folios, then we're at >>>> the point where the mm side of large folios needs to be divorced >>>> from CONFIG_TRANSPARENT_HUGEPAGE and always supported. >>>> Alternatively, CONFIG_TRANSPARENT_HUGEPAGE needs to selected by the >>>> block layer and also every filesystem that wants to support >>>> sector/blocks sizes larger than PAGE_SIZE. IOWs, large folio >>>> support needs to *always* be enabled on systems that say >>>> CONFIG_BLOCK=y. >>> >>> Why CONFIG_BLOCK? I think it is enough if it comes from the FS side >>> right? And for now, the only FS that needs that sort of bs > ps >>> guarantee is XFS with this series. Other filesystems such as bcachefs >>> that call mapping_set_large_folios() only enable it as an optimization >>> and it is not needed for the filesystem to function. >>> >>> So this is my conclusion from the conversation: >>> - Add a dependency in Kconfig on THP for XFS until we fix the dependency >>> of large folios on THP >> >> THP isn't supported on some arches, so isn't this effectively saying XFS can no >> longer be used with those arches, even if the bs <= ps? > > I'm good with that - we're already long past the point where we try > to support XFS on every linux platform. Indeed, we've recent been > musing about making XFS depend on 64 bit only - 32 bit systems don't > have the memory capacity to run the full xfs tool chain (e.g. > xfs_repair) on filesystems over about a TB in size, and they are > greatly limited in kernel memory and vmap areas, both of which XFS > makes heavy use of. Basically, friends don't let friends use XFS on > 32 bit systems, and that's been true for about 20 years now. > > Our problem is the test matrix - if we now have to explicitly test > XFS both with and without large folios enabled to support these > platforms, we've just doubled our test matrix. The test matrix is > already far too large to robustly cover, so anything that requires > doubling the number of kernel configs we have to test is, IMO, a > non-starter. > > That's why we really don't support XFS on 32 bit systems anymore and > why we're talking about making that official with a config option. > If we're at the point where XFS will now depend on large folios (i.e > THP), then we need to seriously consider reducing the supported > arches to just those that support both 64 bit and THP. If niche > arches want to support THP, or enable large folios without the need > for THP, then they can do that work and then they get XFS for > free. > > Just because an arch might run a Linux kernel, it doesn't mean we > have to support XFS on it.... OK. I was just pointing out the impact of adding this Kconfig dependency. If that impact is explicitly considered and desired, then great. I'll leave you to it. Thanks, Ryan > > -Dave.
