On Mo, 2017-04-03 at 13:24 +0100, Mark Cave-Ayland wrote: > On 03/04/17 13:03, Gerd Hoffmann wrote: > > > Hi, > > > >> I checked the branch, is bitmap_copy_and_clear_atomic correct when you > >> have partial updates? Maybe you need to handle partial updates of the > >> first and last word? > > > > Should not be a problem. We might clear some more bits, but these are > > outsize the visible area so they should cause visible corruption (and if > > the visible area changes the display code needs to do a full refresh > > anyway). > > Right. Also is there a reason that > cpu_physical_memory_snapshot_and_clear_dirty() uses BITS_PER_LEVEL when > calculating the alignment used for the first/last addresses?
The code copy doesn't operate on bits but on bitmap longs, so I can just copy the whole long instead of fiddeling with bits. So I round down to a bitmap long start. On 64bit hosts that are units of 64 pages. Actually querying the copied bitmap operates on page granularity of course. > Otherwise you end up expanding the range of your last address > considerably beyond the next page alignment. Yes, it's larger, but as it expands to the start of the long word I expect the penalty is almost zero (same cache line) and being able to just copy the whole long instead of dealing with individual bits should be a win. > And also in the main page > loop why is BITS_PER_LEVEL used? I see that several of the internal > bitmap routines appear to use BITS_PER_LONG for compressing into the > bitmap which might be more appropriate? shift by BITS_PER_LEVEL is the same as division by BITS_PER_LONG cheers, Gerd
