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free to send what you have below and you can add: Reviewed-by: Lorenzo Stoakes On Wed, Nov 26, 2025 at 10:09:36PM +0000, Matthew Wilcox wrote: > On Wed, Nov 26, 2025 at 08:33:47PM +0000, Lorenzo Stoakes wrote: > > On Wed, Nov 26, 2025 at 07:44:17PM +0000, Matthew Wilcox wrote: > > > On Wed, Nov 26, 2025 at 06:55:52PM +0000, Lorenzo Stoakes wrote: > > > > > It's only "impossible" currently due to some fairly esoteric reasoning. > > > > > As far as _this_ function is concerned, it's entirely possible. > > > > > I don't want to leave this trap for the next person who calls > > > > > __vma_enter_locked(TASK_KILLABLE). > > > > > > > > Calls __vma_enter_locked(TASK_KILLABLE) _when detaching_, otherwise > > > > refcount will always be >0. > > > > > > > > So we're only looking at us changing vma_mark_detached() to use > > > > TASK_KILLABLE. > > > > > > > > As this is such a subtle corner case I still think it warrants a > > > > warning. Or at least a VM_WARN_ON_ONCE(1). > > > > > > > > A killable detacher is, as Vlasta points out, kind of an unwise thing to do > > > > anyway right? > > > > > > I missed where that was said? > > > > "Yeah I guess it's for the best to keep vma_mark_detached() use the > > TASK_UNINTERRUPTIBLE variant, maybe document why. Aborting the detaching > > would be counter productive." > > > > https://lore.kernel.org/all/058f5858-f508-40f8-adfe-e5de78621d64@suse.cz/ > > I'm not entirely clear on why aborting a detach is always a bad idea, > but that's part of the MM I don't really understand. Ack insert moany waffle about this impl. being confusing for _everybody_ here :) > > > - A fatal signal arose (assuming nobody ever goes and changes > > rcuwait_wait_event() to add more errors - very likely, not entirely certain > > though, so perhaps 'an error that meant we couldn't wait'.) > > It actually doesn't matter why we got an error. We got an error. > But also the last reader went away. So we're now in a state where we > would not have needed to sleep had we got here half a nanosecond later > than we did. Right, sure. > > > Since you're concerned about the urgency, let me suggest a compromise: > > > > /* > > * We tried waiting on readers, but failed, likely due to a fatal > > * signal arising. Unlock the VMA and check whether the VMA is > > * detached. > > */ > > I think the 'if (err)' is enough to tell the reader that we failed! Yup, but not the unlocking... I can address that later in the series I'm inevitably going to end up sending to improve this file :) > > > if (refcount_sub_and_test(VMA_LOCK_OFFSET, &vma->vm_refcnt)) { > > /* > > * If the VMA is now detached which means we lost a race. > > * Let the caller know the VMA is detached. > > */ > > err = 0; > > } > > > > That gives a _lot_ more information, keeps it relatively top-level, doesn't > > make undue assumptions etc. > > Here's what I now have: > > if (err) { > if (refcount_sub_and_test(VMA_LOCK_OFFSET, &vma->vm_refcnt)) { > /* > * The wait failed, but the last reader went away > * as well. Tell the caller the VMA is detached. > */ > WARN_ON_ONCE(!detaching); > err = 0; > } OK this is fine, respin or ask Andrew to fix-patch and have a tag :) > > > > Are you satisfied with the WARN_ON(!detaching)? > > > > > > > It'd be super weird to reach that code when not detaching so sure, think it > > should be VM_WARN_ON() though since the code would be horribly broken if > > that was not the case already no? > > The other places in this file are WARN_ON_ONCE rather than VM_WARN*, so > keep it consistent. > Fine, sure. Cheers, Lorenzo