On Thu, Apr 18, 2024 at 4:04 AM Jan Kara <jack@xxxxxxx> wrote: > > On Wed 17-04-24 12:33:39, Zach O'Keefe wrote: > > On Wed, Apr 17, 2024 at 4:10 AM Jan Kara <jack@xxxxxxx> wrote: > > > > diff --git a/mm/page-writeback.c b/mm/page-writeback.c > > > > index cd4e4ae77c40a..02147b61712bc 100644 > > > > --- a/mm/page-writeback.c > > > > +++ b/mm/page-writeback.c > > > > @@ -1638,7 +1638,7 @@ static inline void wb_dirty_limits(struct dirty_throttle_control *dtc) > > > > */ > > > > dtc->wb_thresh = __wb_calc_thresh(dtc); > > > > dtc->wb_bg_thresh = dtc->thresh ? > > > > - div_u64((u64)dtc->wb_thresh * dtc->bg_thresh, dtc->thresh) : 0; > > > > + div64_u64(dtc->wb_thresh * dtc->bg_thresh, dtc->thresh) : 0; > ... > > > Thirdly, if thresholds are larger than 1<<32 pages, then dirty balancing is > > > going to blow up in many other spectacular ways - consider only the > > > multiplication on this line - it will not necessarily fit into u64 anymore. > > > The whole dirty limiting code is interspersed with assumptions that limits > > > are actually within u32 and we do our calculations in unsigned longs to > > > avoid worrying about overflows (with occasional typing to u64 to make it > > > more interesting because people expected those entities to overflow 32 bits > > > even on 32-bit archs). Which is lame I agree but so far people don't seem > > > to be setting limits to 16TB or more. And I'm not really worried about > > > security here since this is global-root-only tunable and that has much > > > better ways to DoS the system. > > > > > > So overall I'm all for cleaning up this code but in a sensible way please. > > > E.g. for these overflow issues at least do it one function at a time so > > > that we can sensibly review it. > > > > > > Andrew, can you please revert this patch until we have a better fix? So far > > > it does more harm than good... Thanks! > > > > Shall we just roll-forward with a suitable fix? I think all the > > original code actually "needed" was to cast the ternary predicate, > > like: > > > > ---8<--- > > diff --git a/mm/page-writeback.c b/mm/page-writeback.c > > index fba324e1a010..ca1bfc0c9bdd 100644 > > --- a/mm/page-writeback.c > > +++ b/mm/page-writeback.c > > @@ -1637,8 +1637,8 @@ static inline void wb_dirty_limits(struct > > dirty_throttle_control *dtc) > > * at some rate <= (write_bw / 2) for bringing down wb_dirty. > > */ > > dtc->wb_thresh = __wb_calc_thresh(dtc); > > - dtc->wb_bg_thresh = dtc->thresh ? > > - div64_u64(dtc->wb_thresh * dtc->bg_thresh, dtc->thresh) : 0; > > + dtc->wb_bg_thresh = (u32)dtc->thresh ? > > + div_u64((u64)dtc->wb_thresh * dtc->bg_thresh, dtc->thresh) : 0; > > Well, this would fix the division by 0 but when you read the code you > really start wondering what's going on :) [..] Ya, this was definitely a local fix in an area of code I know very little abit. I stumbled across it in a rather contrived way -- made easier by internal patches -- and felt its existence still warranted a local fix. > [..] And as I wrote above when > thresholds pass UINT_MAX, the dirty limitting code breaks down anyway so I > don't think the machine will be more usable after your fix. Would you be up > for a challenge to modify mm/page-writeback.c so that such huge limits > cannot be set instead? That would be actually a useful fix... :) I can't say my schedule affords me much time to take on any significant unplanned work. Perhaps as a Friday afternoon exercise I'll come back to scope this out, driven by some sense of responsibility garnered from starting down this path ; but ... my TODO list is long. Have a great rest of your day / weekend, Zach > Honza > > > > > /* > > * In order to avoid the stacked BDI deadlock we need > > ---8<--- > > > > Thanks, and apologize for the inconvenience > > > > Zach > > > > > Honza > > > -- > > > Jan Kara <jack@xxxxxxxx> > > > SUSE Labs, CR > > > -- > Jan Kara <jack@xxxxxxxx> > SUSE Labs, CR
