On Thu 11-02-21 14:26:30, Matthew Wilcox wrote: > On Thu, Feb 11, 2021 at 03:20:41PM +0100, Michal Hocko wrote: > > On Thu 11-02-21 13:25:33, Matthew Wilcox wrote: > > > On Thu, Feb 11, 2021 at 02:07:03PM +0100, Michal Hocko wrote: > > > > On Thu 11-02-21 12:57:17, Matthew Wilcox wrote: > > > > > > current->flags should be always manipulated from the user context. But > > > > > > who knows maybe there is a bug and some interrupt handler is calling it. > > > > > > This should be easy to catch no? > > > > > > > > > > Why would it matter if it were? > > > > > > > > I was thinking about a clobbered state because updates to ->flags are > > > > not atomic because this shouldn't ever be updated concurrently. So maybe > > > > a racing interrupt could corrupt the flags state? > > > > > > I don't think that's possible. Same-CPU races between interrupt and > > > process context are simpler because the CPU always observes its own writes > > > in order and the interrupt handler completes "between" two instructions. > > > > I have to confess I haven't really thought the scenario through. My idea > > was to simply add a simple check for an irq context into ->flags setting > > routine because this should never be done in the first place. Not only > > for scope gfp flags but any other PF_ flags IIRC. > > That's not automatically clear to me. There are plenty of places > where an interrupt borrows the context of the task that it happens to > have interrupted. Specifically, interrupts should be using GFP_ATOMIC > anyway, so this doesn't really make a lot of sense, but I don't think > it's necessarily wrong for an interrupt to call a function that says > "Definitely don't make GFP_FS allocations between these two points". Not sure I got your point. IRQ context never does reclaim so anything outside of NOWAIT/ATOMIC is pointless. But you might be refering to a future code where GFP_FS might have a meaning outside of the reclaim context? Anyway if we are to allow modifying PF_ flags from an interrupt contenxt then I believe we should make that code IRQ aware at least. I do not feel really comfortable about async modifications when this is stated to be safe doing in a non atomic way. But I suspect we have drifted away from the original issue. I thought that a simple check would help us narrow down this particular case and somebody messing up from the IRQ context didn't sound like a completely off. -- Michal Hocko SUSE Labs
