On 12 February 2015 at 23:16, Mark Rutland <mark.rutland@xxxxxxx> wrote: > On Thu, Feb 12, 2015 at 02:56:51PM +0000, Ard Biesheuvel wrote: >> On 12 February 2015 at 22:47, Matt Fleming <matt@xxxxxxxxxxxxxxxxxxx> wrote: >> > On Thu, 12 Feb, at 06:39:46PM, Ard Biesheuvel wrote: >> >> >> >> I don't see how doing a single allocation could result in a single >> >> free region to be split into more than 1 occupied region + 2 free >> >> regions. >> >> So no, I don't think it is ... >> > >> > I don't think that's a guarantee we can make, nor is it something we >> > should rely upon. >> > >> > Please explain the user-visible failure that this patch fixes. Does your >> > machine refuse to boot? >> >> I am running UEFI under QEMU and Xen primarily at the moment, and >> experimenting with various build options in Tianocore, One of the >> options is preallocating and freeing blocks of various memory types, >> in a way that should result in the final number of distinct regions to >> be much lower. It could result however in a free memory region to be >> carved up in three instead of two, and that is a failure I have seen >> occur. > > The simple answer is that the machine will fail to boot, beause the > efi_get_memory_map helper will give up after one go, and propagate the > error. The arm-stub will give up when the error is encountered. > Indeed. >> > Why is the 'goto again' loop insufficient in >> > handling this scenario? >> > >> >> Yes, that should solve it as well, so if you prefer I reinstate that, >> I can respin the patch. There is a theoretical possibility that it >> would take more than just one more iteration, but that is highly >> unlikely and it should still always complete. > > Please reinstate the loop. It will make this far less fragile. > Actually, looking again at the original patch, it appears that my analysis was incorrect regarding the possibility that the loop would never terminate. The only thing that could happen if desc_size > sizeof(efi_memory_desc_t) is that you need two iterations instead of one to get a pool allocation that is of sufficient size. So perhaps it is better to just revert the patch. My apologies for the hassle. -- Ard. -- To unsubscribe from this list: send the line "unsubscribe linux-efi" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
