On Thu, Mar 25, 2021 at 07:13:33PM +0100, Thomas Hellström (Intel) wrote: > > On 3/25/21 6:55 PM, Jason Gunthorpe wrote: > > On Thu, Mar 25, 2021 at 06:51:26PM +0100, Thomas Hellström (Intel) wrote: > > > On 3/24/21 9:25 PM, Dave Hansen wrote: > > > > On 3/24/21 1:22 PM, Thomas Hellström (Intel) wrote: > > > > > > We also have not been careful at *all* about how _PAGE_BIT_SOFTW* are > > > > > > used. It's quite possible we can encode another use even in the > > > > > > existing bits. > > > > > > > > > > > > Personally, I'd just try: > > > > > > > > > > > > #define _PAGE_BIT_SOFTW5 57 /* available for programmer */ > > > > > > > > > > > OK, I'll follow your advise here. FWIW I grepped for SW1 and it seems > > > > > used in a selftest, but only for PTEs AFAICT. > > > > > > > > > > Oh, and we don't care about 32-bit much anymore? > > > > On x86, we have 64-bit PTEs when running 32-bit kernels if PAE is > > > > enabled. IOW, we can handle the majority of 32-bit CPUs out there. > > > > > > > > But, yeah, we don't care about 32-bit. :) > > > Hmm, > > > > > > Actually it makes some sense to use SW1, to make it end up in the same dword > > > as the PSE bit, as from what I can tell, reading of a 64-bit pmd_t on 32-bit > > > PAE is not atomic, so in theory a huge pmd could be modified while reading > > > the pmd_t making the dwords inconsistent.... How does that work with fast > > > gup anyway? > > It loops to get an atomic 64 bit value if the arch can't provide an > > atomic 64 bit load > > Hmm, ok, I see a READ_ONCE() in gup_pmd_range(), and then the resulting pmd > is dereferenced either in try_grab_compound_head() or __gup_device_huge(), > before the pmd is compared to the value the pointer is currently pointing > to. Couldn't those dereferences be on invalid pointers? Uhhhhh.. That does look questionable, yes. Unless there is some tricky reason why a 64 bit pmd entry on a 32 bit arch either can't exist or has a stable upper 32 bits.. The pte does it with ptep_get_lockless(), we probably need the same for the other levels too instead of open coding a READ_ONCE? Jason _______________________________________________ dri-devel mailing list dri-devel@xxxxxxxxxxxxxxxxxxxxx https://lists.freedesktop.org/mailman/listinfo/dri-devel
