On 11/30/2016 06:21 AM, Yuriy Kolerov wrote: >> On Tue 29-11-16 18:29:06, Yuriy Kolerov wrote: >>> > > Despite the fact that subtraction of unsigned integers is a defined >>> > > behaviour however such operations can lead to unexpected results. Thus >>> > > it is better to check both left and right boundaries to avoid >>> > > potential bugs as it done in the generic page.h. >> > >> > Why and which code would use an out of range pfn? Why other arches do >> > not need to care? > Actually some arches do care about checking of both left and right boundaries (e.g. avr32, sparc, etc). The problem is that a value of pfn may be calculated incorrectly in some places of the kernel. E.g. not long ago I sent a patch which fixes truncation of the most significant byte in pfn/pte in some cases (in the kernel with PAE40, however it is not a FLATMEM case). So such situations can happens in the most unexpected places. > So the point is - is this a preventive fix (desired thing) or it being there would have helped find the PAE40 bug earlier / easier. Woudl it have prevented the kernel crash. If so then this is a nobrainer fix. BTW did you try to gauge the code gen impact - this function gets pulled all over the place in mm code. So build kernel with and w/o change and do a scripts/bloat-o-meter -Vineet
