[<prev] [next>] [<thread-prev] [thread-next>] [day] [month] [year] [list]
Message-ID: <20220107033256.GJ7074@brightrain.aerifal.cx>
Date: Thu, 6 Jan 2022 22:32:56 -0500
From: Rich Felker <dalias@...c.org>
To: Colin Cross <ccross@...gle.com>
Cc: musl@...ts.openwall.com
Subject: Re: [PATCH] Add mallinfo2 and mallinfo
On Thu, Jan 06, 2022 at 03:42:52PM -0800, Colin Cross wrote:
> On Thu, Jan 6, 2022 at 2:00 PM Rich Felker <dalias@...c.org> wrote:
> >
> > On Thu, Jan 06, 2022 at 12:37:09PM -0800, Colin Cross wrote:
> > > glibc introduced mallinfo2 [1], which solves some of the arguments [2]
> > > against including mallinfo in musl by expanding the width of the
> > > returned counters from int to size_t.
> > >
> > > This patch implements mallinfo2 without requiring any additional
> > > metadata. It iterates through the meta_areas and metas in order
> > > to count mmap, large and small allocations, and produces ordblks,
> > > hblks, hblkhd, uordblks and fordblks values.
> > >
> > > Once mallinfo2 exists, it is trivial to implement mallinfo that caps
> > > the mallinfo2 outputs such that they fit in the int fields returned
> > > by mallinfo.
> > >
> > > [1] https://sourceware.org/git/?p=glibc.git;a=commit;h=e3960d1c57e57f33e0e846d615788f4ede73b945
> > > [2] https://www.openwall.com/lists/musl/2018/01/17/2
> >
> > Historically, mallinfo was omitted intentionally in musl partly
> > because of the wrong-types issue (fixed by mallinfo2), but also partly
> > because the set of data items returned is built around certain
> > assumptions about the malloc implementation that aren't necessarily
> > valid, especially for our allocators. This could be revisited, but I'm
> > not sure we'll find good justification to add it.
>
> Many of the mallinfo fields are meaningless and left zero. I left
> arenas and keepcost zero because mallocng never puts allocations on
> the heap, only metadata. Similarly, all the fastbin-related fields
> are zero because fastbin seems very specific to glibc's
> implementation. The remainder seem to map to mallocng fairly well:
> hblks and hblkhd track the number and total size of mmap allocations,
> uordblks tracks the total size of all allocations, ordblks and
> fordblks track the number and total size of the free slots.
Here I'd tend to disagree and I think this highlights how the model
doesn't map well. I would not call the metadata "the heap" just
because it's (when possible) obtained with brk. I would call "the
heap" the storage that's managed by the allocator's bookkeeping for
reuse without going through a per-allocation syscall to allocate and
free it, and distinguish that only from individually-mmapped units
that are guaranteed to be unmapped when freed.
> > > The motivation for this patch is an attempt to use musl instead of glibc
> > > to build host tools used when building the Android platform and the
> > > tools that are distributed to app developers as part of the Android SDK.
> > > mallinfo is used in a variety of third-party code built as part of
> > > building Android, and tests and benchmarks in the Android tree.
> > >
> > > The implementation has been lightly tested with bionic's malloc.mallinfo
> > > and malloc.mallinfo2 tests, which verify that a variety of different
> > > allocation sizes result in an increase of the uordblks value by at
> > > least the usable size of the returned allocation.
> > >
> > > I can keep this as a local patch in Android if it is still not acceptable
> > > for musl.
> >
> > Is there a reason not to just #ifdef HAVE_MALLINFO it out, or do a
> > dummy implementation, or one that makes up semi-reasonable numbers
> > purely based on /proc/self/maps without poking at malloc internals?
>
> It's often used in tests and benchmarks to verify that calling the
> method repeatedly doesn't leak memory. Given how easy it was to
> implement I'd probably keep this implementation in Android rather than
> try to deduce it from /proc/self/maps, which could contain many
> non-malloc anonymous pages and can't tell the difference between
> allocated and freed memory.
For testing lack of memory leak, I'd think "whole process vm size"
would be more meaningful than something from mallinfo. For example
mallinfo wouldn't catch accidentally re-mmapping a file each time it's
used without unmapping it, or manual mmap of MAP_ANON.
> Somewhat relatedly, some patches I wrote for the kernel in Android to
> allow naming anonymous memory regions have finally gone into the
> linux-next branch [1]. If that becomes widely enabled then musl's
> allocator could tag mmap regions, which would provide more information
> for parsing stats out of /proc/self/maps. It still couldn't tell the
> difference between allocated and free though.
I don't think this is something we'd do; it would significantly
increase runtime cost. (Keep in mind mallocng's groups are not
intended to necessarily be long-lived.)
> > > diff --git a/src/malloc/mallocng/mallinfo.c b/src/malloc/mallocng/mallinfo.c
> > > new file mode 100644
> > > index 00000000..c60840b1
> > > --- /dev/null
> > > +++ b/src/malloc/mallocng/mallinfo.c
> > > @@ -0,0 +1,73 @@
> > > +#include <limits.h>
> > > +#include <malloc.h>
> > > +#include <stddef.h>
> > > +
> > > +#include "glue.h"
> > > +#include "meta.h"
> > > +
> > > +static void accumulate_meta(struct mallinfo2 *mi, struct meta *g) {
> > > + int sc = g->sizeclass;
> > > + if (sc >= 48) {
> > > + // Large mmap allocation
> > > + mi->hblks++;
> > > + mi->uordblks += g->maplen*4096;
> > > + mi->hblkhd += g->maplen*4096;
> > > + } else {
> > > + if (g->freeable && !g->maplen) {
> > > + // Small size slots are embedded in a larger slot, avoid double counting
> > > + // by subtracing the size of the larger slot from the total used memory.
> > > + struct meta* outer_g = get_meta((void*)g->mem);
> > > + int outer_sc = outer_g->sizeclass;
> > > + int outer_sz = size_classes[outer_sc]*UNIT;
> > > + mi->uordblks -= outer_sz;
> > > + }
> > > + int sz = size_classes[sc]*UNIT;
> > > + int mask = g->avail_mask | g->freed_mask;
> > > + int nr_unused = __builtin_popcount(mask);
> > > + mi->ordblks += nr_unused;
> > > + mi->uordblks += sz*(g->last_idx+1-nr_unused);
> > > + mi->fordblks += sz*nr_unused;
> > > + }
> > > +}
> >
> > For upstreaming, __builtin_popcount wouldn't be usable. But aside from
> > that, the approach here looks roughly correct. I don't see any
> > correction for the case where a g->last_idx==1 and sc<48, in which
> > case it's possible that map_len is less than the length for the size
> > class. These should probably be treated like "individually mmapped"
> > allocations. This is one place where trying to fit the mallinfo data
> > model with an allocator that doesn't match its assumptions is
> > something of a hack.
>
> I'll take a look at this, I assume you mean when g->last_idx==0?
Yes. If g->last_idx==0, you need to use g->maplen to determine the
size. Whether that's better counted as "individually mmapped" or
"heap" I'm not sure.
> > > +static void accumulate_meta_area(struct mallinfo2 *mi, struct meta_area *ma) {
> > > + for (int i=0; i<ma->nslots; i++) {
> > > + if (ma->slots[i].mem) {
> > > + accumulate_meta(mi, &ma->slots[i]);
> > > + }
> > > + }
> > > +}
> > > +
> > > +struct mallinfo2 mallinfo2() {
> > > + struct mallinfo2 mi = {0};
> > > +
> > > + rdlock();
> > > + struct meta_area *ma = ctx.meta_area_head;
> > > + while (ma) {
> > > + accumulate_meta_area(&mi, ma);
> > > + ma = ma->next;
> > > + }
> > > + unlock();
> > > +
> > > + return mi;
> > > +}
> > > +
> > > +#define cap(x) ((x > INT_MAX) ? INT_MAX : x)
> > > +
> > > +struct mallinfo mallinfo() {
> > > + struct mallinfo mi = {0};
> > > + struct mallinfo2 mi2 = mallinfo2();
> > > +
> > > + mi.arena = cap(mi2.arena);
> > > + mi.ordblks = cap(mi2.ordblks);
> > > + mi.smblks = cap(mi2.smblks);
> > > + mi.hblks = cap(mi2.hblks);
> > > + mi.hblkhd = cap(mi2.hblkhd);
> > > + mi.usmblks = cap(mi2.usmblks);
> > > + mi.fsmblks = cap(mi2.fsmblks);
> > > + mi.uordblks = cap(mi2.uordblks);
> > > + mi.fordblks = cap(mi2.fordblks);
> > > + mi.keepcost = cap(mi2.keepcost);
> > > +
> > > + return mi;
> > > +}
> > > --
> > > 2.34.1.448.ga2b2bfdf31-goog
> >
> > If the API is added upstream, it really should be provided by both
> > mallocng and oldmalloc, with the legacy mallinfo (int) wrapper, if
> > any, in src/malloc rather than src/malloc/mallocng. Available
> > functions should not differ based on --with-malloc choice.
>
> I can add the other implementations if this is likely to be accepted,
> if I'm keeping this in Android I won't bother.
Indeed, this is only relevant if it's adopted as public API.
Rich
Powered by blists - more mailing lists
Confused about mailing lists and their use?
Read about mailing lists on Wikipedia
and check out these
guidelines on proper formatting of your messages.
