On Wed, Jan 15, 2025 at 4:59 PM Darrick J. Wong <djwong@xxxxxxxxxx> wrote:
>
> On Wed, Jan 15, 2025 at 04:47:30PM -0800, Joanne Koong wrote:
> > On Wed, Jan 15, 2025 at 1:37 PM Darrick J. Wong <djwong@xxxxxxxxxx> wrote:
> > >
> > > On Wed, Jan 15, 2025 at 10:31:06AM -0800, Joanne Koong wrote:
> > > > Add support for reads/writes from buffers backed by hugepages.
> > > > This can be enabled through the '-h' flag. This flag should only be used
> > > > on systems where THP capabilities are enabled.
> > > >
> > > > This is motivated by a recent bug that was due to faulty handling of
> > > > userspace buffers backed by hugepages. This patch is a mitigation
> > > > against problems like this in the future.
> > > >
> > > > Signed-off-by: Joanne Koong <joannelkoong@xxxxxxxxx>
> > > > Reviewed-by: Brian Foster <bfoster@xxxxxxxxxx>
> > > > ---
> > > > ltp/fsx.c | 119 +++++++++++++++++++++++++++++++++++++++++++++++++-----
> > > > 1 file changed, 108 insertions(+), 11 deletions(-)
> > > >
> > > > diff --git a/ltp/fsx.c b/ltp/fsx.c
> > > > index 41933354..8d3a2e2c 100644
> > > > --- a/ltp/fsx.c
> > > > +++ b/ltp/fsx.c
> > > > @@ -190,6 +190,7 @@ int o_direct; /* -Z */
> > > > int aio = 0;
> > > > int uring = 0;
> > > > int mark_nr = 0;
> > > > +int hugepages = 0; /* -h flag */
> > > >
> > > > int page_size;
> > > > int page_mask;
> > > > @@ -2471,7 +2472,7 @@ void
> > > > usage(void)
> > > > {
> > > > fprintf(stdout, "usage: %s",
> > > > - "fsx [-dfknqxyzBEFHIJKLORWXZ0]\n\
> > > > + "fsx [-dfhknqxyzBEFHIJKLORWXZ0]\n\
> > > > [-b opnum] [-c Prob] [-g filldata] [-i logdev] [-j logid]\n\
> > > > [-l flen] [-m start:end] [-o oplen] [-p progressinterval]\n\
> > > > [-r readbdy] [-s style] [-t truncbdy] [-w writebdy]\n\
> > > > @@ -2484,6 +2485,7 @@ usage(void)
> > > > -e: pollute post-eof on size changes (default 0)\n\
> > > > -f: flush and invalidate cache after I/O\n\
> > > > -g X: write character X instead of random generated data\n\
> > > > + -h hugepages: use buffers backed by hugepages for reads/writes\n\
> > >
> > > If this requires MADV_COLLAPSE, then perhaps the help text shouldn't
> > > describe the switch if the support wasn't compiled in?
> > >
> > > e.g.
> > >
> > > -g X: write character X instead of random generated data\n"
> > > #ifdef MADV_COLLAPSE
> > > " -h hugepages: use buffers backed by hugepages for reads/writes\n"
> > > #endif
> > > " -i logdev: do integrity testing, logdev is the dm log writes device\n\
> > >
> > > (assuming I got the preprocessor and string construction goo right; I
> > > might be a few cards short of a deck due to zombie attack earlier)
> >
> > Sounds great, I'll #ifdef out the help text -h line. Hope you feel better.
> > >
> > > > -i logdev: do integrity testing, logdev is the dm log writes device\n\
> > > > -j logid: prefix debug log messsages with this id\n\
> > > > -k: do not truncate existing file and use its size as upper bound on file size\n\
> > [...]
> > > > +}
> > > > +
> > > > +#ifdef MADV_COLLAPSE
> > > > +static void *
> > > > +init_hugepages_buf(unsigned len, int hugepage_size, int alignment)
> > > > +{
> > > > + void *buf;
> > > > + long buf_size = roundup(len, hugepage_size) + alignment;
> > > > +
> > > > + if (posix_memalign(&buf, hugepage_size, buf_size)) {
> > > > + prterr("posix_memalign for buf");
> > > > + return NULL;
> > > > + }
> > > > + memset(buf, '\0', buf_size);
> > > > + if (madvise(buf, buf_size, MADV_COLLAPSE)) {
> > >
> > > If the fsx runs for a long period of time, will it be necessary to call
> > > MADV_COLLAPSE periodically to ensure that reclaim doesn't break up the
> > > hugepage?
> > >
> >
> > imo, I don't think so. My understanding is that this would be a rare
> > edge case that happens when the system is constrained on memory, in
> > which case subsequent calls to MADV_COLLAPSE would most likely fail
> > anyways.
>
> Hrmmm... well I /do/ like to run memory constrained VMs to prod reclaim
> into stressing the filesystem more. But I guess there's no good way for
> fsx to know that something happened to it. Unless there's some even
> goofier way to force a hugepage, like shmem/hugetlbfs (ugh!) :)
I can't think of a better way to force a hugepage either. I believe
shmem and hugetlbfs would both require root privileges to do so, and
if i'm not mistaken, shmem hugepages are still subject to being broken
up by reclaim.
>
> Will have to ponder hugepage renewasl -- maybe we should madvise every
> few thousand fsxops just to be careful?
I can add this in, but on memory constrained VMs, would this be
effective? To me, it seems like in the majority of cases, subsequent
attempts at collapsing the broken pages back into a hugepage would
fail due to memory still being constrained. In which case, I guess
we'd exit the test altogether? It kind of seems to me like if the user
wants to test out hugepages functionality of their filesystem, then
the onus is on them to run the test in an environment that can
adequately and consistently support hugepages.
Thanks,
Joanne
>
> --D
>
> >
> > Thanks,
> > Joanne
> >
> > > > + prterr("madvise collapse for buf");
> > > > + free(buf);
> > > > + return NULL;
> > > > + }
> > > > +
> > > > + return buf;
> > > > +}
> > > > +#else
> > > > +static void *
> > > > +init_hugepages_buf(unsigned len, int hugepage_size, int alignment)
> > > > +{
> > > > + return NULL;
> > > > +}
> > > > +#endif
> > > > +
> > > > +static void
> > > > +init_buffers(void)
> > > > +{
> > > > + int i;
> > > > +
> > > > + original_buf = (char *) malloc(maxfilelen);
> > > > + for (i = 0; i < maxfilelen; i++)
> > > > + original_buf[i] = random() % 256;
> > > > + if (hugepages) {
> > > > + long hugepage_size = get_hugepage_size();
> > > > + if (hugepage_size == -1) {
> > > > + prterr("get_hugepage_size()");
> > > > + exit(102);
> > > > + }
> > > > + good_buf = init_hugepages_buf(maxfilelen, hugepage_size, writebdy);
> > > > + if (!good_buf) {
> > > > + prterr("init_hugepages_buf failed for good_buf");
> > > > + exit(103);
> > > > + }
> > > > +
> > > > + temp_buf = init_hugepages_buf(maxoplen, hugepage_size, readbdy);
> > > > + if (!temp_buf) {
> > > > + prterr("init_hugepages_buf failed for temp_buf");
> > > > + exit(103);
> > > > + }
> > > > + } else {
> > > > + unsigned long good_buf_len = maxfilelen + writebdy;
> > > > + unsigned long temp_buf_len = maxoplen + readbdy;
> > > > +
> > > > + good_buf = calloc(1, good_buf_len);
> > > > + temp_buf = calloc(1, temp_buf_len);
> > > > + }
> > > > + good_buf = round_ptr_up(good_buf, writebdy, 0);
> > > > + temp_buf = round_ptr_up(temp_buf, readbdy, 0);
> > > > +}
> > > > +
> > > > static struct option longopts[] = {
> > > > {"replay-ops", required_argument, 0, 256},
> > > > {"record-ops", optional_argument, 0, 255},
> > > > @@ -2883,7 +2980,7 @@ main(int argc, char **argv)
> > > > setvbuf(stdout, (char *)0, _IOLBF, 0); /* line buffered stdout */
> > > >
> > > > while ((ch = getopt_long(argc, argv,
> > > > - "0b:c:de:fg:i:j:kl:m:no:p:qr:s:t:uw:xyABD:EFJKHzCILN:OP:RS:UWXZ",
> > > > + "0b:c:de:fg:hi:j:kl:m:no:p:qr:s:t:uw:xyABD:EFJKHzCILN:OP:RS:UWXZ",
> > > > longopts, NULL)) != EOF)
> > > > switch (ch) {
> > > > case 'b':
> > > > @@ -2916,6 +3013,14 @@ main(int argc, char **argv)
> > > > case 'g':
> > > > filldata = *optarg;
> > > > break;
> > > > + case 'h':
> > > > + #ifndef MADV_COLLAPSE
> > >
> > > Preprocessor directives should start at column 0, like most of the rest
> > > of fstests.
> > >
> > > --D
> > >
