On Tue, 7 Nov 2017 17:01:58 +0300 "Kirill A. Shutemov" <kirill@xxxxxxxxxxxxx> wrote: > On Tue, Nov 07, 2017 at 07:15:58PM +0530, Aneesh Kumar K.V wrote: > > > > > > > > If it is decided to keep these kind of heuristics, can we get just a > > > small but reasonably precise description of each change to the > > > interface and ways for using the new functionality, such that would be > > > suitable for the man page? I couldn't fix powerpc because nothing > > > matches and even Aneesh and you differ on some details (MAP_FIXED > > > behaviour). > > > > > > I would consider MAP_FIXED as my mistake. We never discussed this explicitly > > and I kind of assumed it to behave the same way. ie, we search in lower > > address space (128TB) if the hint addr is below 128TB. > > > > IIUC we agree on the below. > > > > 1) MAP_FIXED allow the addr to be used, even if hint addr is below 128TB but > > hint_addr + len is > 128TB. > > > > 2) For everything else we search in < 128TB space if hint addr is below > > 128TB > > > > 3) We don't switch to large address space if hint_addr + len > 128TB. The > > decision to switch to large address space is primarily based on hint addr > > > > Is there any other rule we need to outline? Or is any of the above not > > correct? > > That's correct. > Thanks guys, I'll send out some powerpc patches to match -- it deviates in its MAP_FIXED handling (treats it the same as !MAP_FIXED). So these semantics are what we're going with? Anything that does mmap() is guaranteed of getting a 47-bit pointer and it can use the top 17 bits for itself? Is intended to be cross-platform or just x86 and power specific? Also, this may follow from deduction from 1-3, but for explicit specification in man page: 4) To get an unspecified allocation with the largest possible address range, we pass in -1 for mmap hint. Are we allowing 8 bits bits of unused address in this case, or must the app not assume anything about number of bits used? Thanks, Nick
