On 12/19/24 18:26, David Hildenbrand wrote: > On 19.12.24 18:14, Shakeel Butt wrote: >> On Thu, Dec 19, 2024 at 05:41:36PM +0100, David Hildenbrand wrote: >>> On 19.12.24 17:40, Shakeel Butt wrote: >>>> On Thu, Dec 19, 2024 at 05:29:08PM +0100, David Hildenbrand wrote: >>>> [...] >>>>>> >>>>>> If you check the code just above this patch, this >>>>>> mapping_writeback_indeterminate() check only happen for pages under >>>>>> writeback which is a temp state. Anyways, fuse folios should not be >>>>>> unmovable for their lifetime but only while under writeback which is >>>>>> same for all fs. >>>>> >>>>> But there, writeback is expected to be a temporary thing, not >>>>> possibly: >>>>> "AS_WRITEBACK_INDETERMINATE", that is a BIG difference. >>>>> >>>>> I'll have to NACK anything that violates ZONE_MOVABLE / ALLOC_CMA >>>>> guarantees, and unfortunately, it sounds like this is the case >>>>> here, unless >>>>> I am missing something important. >>>>> >>>> >>>> It might just be the name "AS_WRITEBACK_INDETERMINATE" is causing >>>> the confusion. The writeback state is not indefinite. A proper fuse fs, >>>> like anyother fs, should handle writeback pages appropriately. These >>>> additional checks and skips are for (I think) untrusted fuse servers. >>> >>> Can unprivileged user space provoke this case? >> >> Let's ask Joanne and other fuse folks about the above question. >> >> Let's say unprivileged user space can start a untrusted fuse server, >> mount fuse, allocate and dirty a lot of fuse folios (within its dirty >> and memcg limits) and trigger the writeback. To cause pain (through >> fragmentation), it is not clearing the writeback state. Is this the >> scenario you are envisioning? > > Yes, for example causing harm on a shared host (containers, ...). > > If it cannot happen, we should make it very clear in documentation and > patch descriptions that it can only cause harm with privileged user > space, and that this harm can make things like CMA allocations, memory > onplug, ... fail, which is rather bad and against concepts like > ZONE_MOVABLE/MIGRATE_CMA. > > Although I wonder what would happen if the privileged user space daemon > crashes (e.g., OOM killer?) and simply no longer replies to any messages. > The request is canceled then - that should clear the page/folio state I start to wonder if we should introduce really short fuse request timeouts and just repeat requests when things have cleared up. At least for write-back requests (in the sense that fuse-over-network might be slow or interrupted for some time). Thanks, Bernd
