On Tue, Apr 03, 2018 at 03:30:46PM -0700, Andrew Morton wrote: > On Sun, 1 Apr 2018 22:35:06 -0400 jglisse@xxxxxxxxxx wrote: > > > From: Ralph Campbell <rcampbell@xxxxxxxxxx> > > > > Use of pte_write(pte) is only valid for present pte, the common code > > which set the migration entry can be reach for both valid present > > pte and special swap entry (for device memory). Fix the code to use > > the mpfn value which properly handle both cases. > > > > On x86 this did not have any bad side effect because pte write bit > > is below PAGE_BIT_GLOBAL and thus special swap entry have it set to > > 0 which in turn means we were always creating read only special > > migration entry. > > Does this mean that the patch only affects behaviour of non-x86 systems? No it affect x86 as explained below (ie it forces a second page fault). > > > So once migration did finish we always write protected the CPU page > > table entry (moreover this is only an issue when migrating from device > > memory to system memory). End effect is that CPU write access would > > fault again and restore write permission. > > That sounds a bit serious. Was a -stable backport considered? Like discuss previously with Michal, for lack of upstream user yet (and PowerPC users of this code are not upstream either yet AFAIK). Once i get HMM inside nouveau upstream, i will evaluate if people wants all fixes to be back ported to stable. Finaly this one isn't too bad, it just burn CPU cycles by forcing CPU to take a second fault on write access ie double fault the same address. There is no corruption or incorrect states (it behave as a COWed page from a fork with a mapcount of 1). Do you still want me to be more aggressive with stable backport ? I don't mind either way. I expect to get HMM nouveau upstream over next couple release cycle. Cheers, Jérôme
