On Wed, Oct 31, 2012 at 3:33 AM, Michel Lespinasse <walken@xxxxxxxxxx> wrote: > My own feel for this series is that I'm fairly confident in the > robustness of my vm_unmapped_area() implementation; however I would > like to confirm that people are happy with this new interface. Also > the code that figures out what constraints to pass to > vm_unmapped_area() is a bit odd; I have tried to make the constraints > match the behavior of the current code but it's not clear to me if > that behavior makes sense in the first place. I wanted to expand a bit on that by listing some of these behaviors I have made sure to preserve without really understanding why they are as they are: - arch_get_unmapped_area() doesn't make use of mm->mmap_base, this value is used only when doing downwards allocations. However, many architectures including x86_64 carefully initialize this (in arch_pick_mmap_layout() ) to different values based on the up/down allocation direction. It seems that the legacy (upwards allocation) mmap_base value is irrelevant as I don't see any place using it ??? - For downwards allocations, it is not clear if the lowest valid address should be 0 or PAGE_SIZE. Existing brute-force search code will treat address 0 as valid on entering the loop, but invalid when reaching the end of the loop. - When user passes a suggested address without the MAP_FIXED flag, the address range we validate the address against varies depending on the upwards/downwards allocation direction. This doesn't make much sense since there is no address space search taking place in this case. - The stragegy of allocating upwards if the downwards allocation failed is a bit strange. I'm not sure what we really want; maybe we only need to extend the valid address range for the initial search ? (IIRC Rik's initial patch series got rid of this redundant search, but didn't explain why this was considered safe). That's all I noticed, but this is really most of the remaining code left in arch_get_unmapped_area[_topdown]... and I didn't even go into architectures other than x86, where I could find some additional questionable stuff (but I don't even want to go there before we at least agree on the general principle of this patch series). I hope with a proper understanding of the allocation strategies / constraints it might be possible to unify the remaining arch_get_unmapped_area[_topdown] code between architectures, but I'm keeping this for a later step as I'm obviously not informed enough to tackle that just yet... -- Michel "Walken" Lespinasse A program is never fully debugged until the last user dies. -- To unsubscribe, send a message with 'unsubscribe linux-mm' in the body to majordomo@xxxxxxxxx. For more info on Linux MM, see: http://www.linux-mm.org/ . Don't email: <a href=mailto:"dont@xxxxxxxxx";> email@xxxxxxxxx </a>