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Note that you may >> need to increase your RLIMIT_NOFILE before running the program. >>=20 >> int main() { >> int pipefd[2]; >> for (int i =3D 0; i < 1025; i++) >> if (pipe(pipefd) =3D=3D -1) >> return 1; >> size_t bufsz =3D fcntl(pipefd[1], F_GETPIPE_SZ); >> printf("%zd\n", bufsz); >> char *buf =3D calloc(bufsz, 1); >> write(pipefd[1], buf, bufsz); >> read(pipefd[0], buf, bufsz-1); >> write(pipefd[1], buf, 1); >> } >>=20 >> Signed-off-by: Alex Xu (Hello71) >> --- >=20 > Is this due to the changes that happened in 5.5? If so, a cc: stable > and a fixes tag would be nice to have :) >=20 >> See discussion at https://lore.kernel.org/lkml/1628086770.5rn8p04n6j.non= e@localhost/. >=20 > This can go up in the changelog text too. >=20 > thanks, >=20 > greg k-h >=20 I tested 5.4 and it exhibits the same problem as master using this=20 non-racy program. I think the problem goes back to v4.5, the first=20 release with 759c01142a ("pipe: limit the per-user amount of pages=20 allocated in pipes"). The issue likely become more apparent with the=20 improvement in pipe performance from v5.5, whereas before that, pipes=20 were too slow for the issue to manifest in racy environments. I'll send a new patch with #include lines and a Fixes: 759c01142a. I'm=20 not 100% sure that it actually goes back that far, but the worst thing=20 that can plausibly happen is that applications opening very large=20 numbers of pipes suddenly use slightly more memory. I certainly hope=20 nobody is relying on pipes randomly blocking roughly 1/4096 of the=20 time. Regards, Alex.