Re: [PATCH] selftests/bpf: Page out as late a possible in file_reader

[Shung-Hsi Yu <shung-hsi.yu@suse.com>]

On Tue, Apr 21, 2026 at 07:59:08AM +0200, Jerome Marchand wrote:
> On 4/20/26 6:52 PM, Mykyta Yatsenko wrote:
> > On 4/20/26 2:46 PM, Jerome Marchand wrote:
> > > The file_reader/on_open_expect_fault fails consistently on my system.
> > > It expects a page fault on first dynptr read of some range the exe
> > > file of the current process because it has paged out that page range
> > > earlier. However a lot can happen to that range (which depending on
> > > the actual memory layout could contain text section, data section,
> > > sections )related to dynamic linking...) between the moment it was
> > > paged out and the moment the bpf program expected to hit a pagefault
> > > actually run.
> > > 
> > > A bit of instrumentation with mincore() shows that pages from that
> > > range were accessed several times before the program is run. In
> > > particular the call of file_reader__load() seems to fault all the
> > > range in.
> > > 
> > > Move the call to madvise(MADV_PAGEOUT) to just before attaching the
> > > program to minimize the risk of having those page pulled back in from
> > > under our feet.
> > > 
> > > Signed-off-by: Jerome Marchand <jmarchan@redhat.com>
> > > ---
> > 
> > Thank you for the patch, the change looks good. Does it fail
> > consistently on 4K page size?
> 
> It did when I ran the test manually. On our automated testing system, it
> failed intermittently.

On my fork of BPF CI the test was failing[1] consistently, even when
this commit is applied. However I wasn't able to reproduce locally on my
laptop when I ran manually.

Any hints regarding where to look and how to reproduce?

Thanks,
Shung-Hsi

1: https://github.com/shunghsiyu/libbpf/actions/runs/26807198259/job/79027625188

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