Re: [PATCH 0/1] fsmonitor: skip sanity check if the index is split

[Utsav Shah <ukshah2@illinois.edu>]

On Mon, Nov 11, 2019 at 8:55 AM Kevin Willford
<Kevin.Willford@microsoft.com> wrote:
>
> > From: git-owner@vger.kernel.org <git-owner@vger.kernel.org> On Behalf
> > Of Junio C Hamano
> > Sent: Sunday, November 10, 2019 7:01 PM
> >
> > Junio C Hamano <gitster@pobox.com> writes:
> >
> > > "Utsav Shah via GitGitGadget" <gitgitgadget@gmail.com> writes:
> > >
> > >> At the very least, this patch mitigates an over-eager check for split
> > >> index users while maintaining good invariants for the standard case.
> > >
> > > OK, it sounds more like this "it does not make any sense to compare
> > > the position in the fsmonitor bitmap (which covers the entire thing)
> > > with the position in just a split part of the index (which covers only
> > > the delta over the base index)"?  If that is the case, it means that
> > > the "check" is even worse than being "over-eager"---it simply is not
> > > correct.
> >
> > Having said all that, I wonder if we are doing the right thing with or without
> > 3444ec2e ("fsmonitor: don't fill bitmap with entries to be removed", 2019-10-
> > 11) in the split-index mode in the first place.
> >
> > The fact that your "loosen the check and allow 'pos' that identifies a tracked
> > path used by the fsmonitor bitmap to be larger than the size of the istate-
> > >cache[]" patch under discussion is needed is that 'pos' may sometimes be
> > larger than isate->cache[] no?  Then what happens in this hunk, for example?
> >
> > diff --git a/fsmonitor.c b/fsmonitor.c
> > index 231e83a94d..1f4aa1b150 100644
> > --- a/fsmonitor.c
> > +++ b/fsmonitor.c
> > @@ -14,8 +14,13 @@ struct trace_key trace_fsmonitor =
> > TRACE_KEY_INIT(FSMONITOR);  static void fsmonitor_ewah_callback(size_t
> > pos, void *is)  {
> >       struct index_state *istate = (struct index_state *)is;
> > -     struct cache_entry *ce = istate->cache[pos];
> > +     struct cache_entry *ce;
> >
> > +     if (pos >= istate->cache_nr)
> > +             BUG("fsmonitor_dirty has more entries than the index
> > (%"PRIuMAX" >= %u)",
> > +                 (uintmax_t)pos, istate->cache_nr);
> > +
> > +     ce = istate->cache[pos];
> >       ce->ce_flags &= ~CE_FSMONITOR_VALID;
> >
> > The istate->cache[] is a dynamic array whose size is managed via the usual
> > ALLOC_GROW() using istate->cache_nr and istate->cache_alloc, whether the
> > split-index feature is in use.  When your patch makes a difference, then,
> > doesn't the access to istate->cache[] pick up a random garbage and then flip
> > the bit?
> >
> > Puzzled...  In any case, "check is worse than over-eager, it simply is wrong" I
> > wrote in the message I am responding to is totally incorrect, it seems.  It
> > smells like lifting the check would just hide the underlying problem under the
> > rug?
>
> I agree.  The only 2 places that excluding the split-index make sense are in
> read_fsmonitor_extension and write_fsmonitor_extension because the
> index_state that is being passing into those methods could be the delta index
> in which case the number of entries for the fsmonitor bitmap would almost
> always be more and cause the BUG to be hit which it should not be.
>
> The reason it is not needed and should not be in the other 2 places is they
> are ran from tweak_fsmonitor which is ran at post_read_index_from which
> is after the base and delta indexes have been loaded into the indes_state and
> the index_state will have all the entries and if the fsmonitor bitmap is bigger
> than the number of entries then the BUG should be hit.

Thanks. What exactly is the delta index? Is it the "split" index, vs
the shared indices? I was surprised to see cache_nr being zero. My
understanding was that cache and cache_nr would always be the
materialized version of the entire index, which is clearly incorrect.

>
> Kevin