Re: [LTP] [PATCH v1] mmap21.c: Test for new MAP_DROPPABLE flag for mmap

[Wei Gao via ltp <ltp@lists.linux.it>]

On Tue, Dec 31, 2024 at 06:04:54PM +0800, Li Wang wrote:
> On Sat, Dec 28, 2024 at 9:32 PM Wei Gao via ltp <ltp@lists.linux.it> wrote:
> 
> > Signed-off-by: Wei Gao <wegao@suse.com>
> > ---
> 
> +               for (size_t i = 0; i < alloc_size; i += page_size) {
> > +                       if (!*(uint8_t *)(alloc + i)) {
> > +                               *shared_var = 1;
> > +                               break;
> > +                       }
> > +               }
> > +       }
> > +
> > +       TST_EXP_EQ_LI((*shared_var), 1);
> >
> 
> The test assumes that reclaimed MAP_DROPPABLE pages will be
> zeroed out when accessed. While this behavior is plausible, it depends
> on how MAP_DROPPABLE is implemented in the kernel.
> 
> Do you have any clue (documents or code) that indicates this is true?

Thanks for your quick feedback , i will try to give clue for this:

1) Base the kernel commit message indicate memory will be zero.

commit 9651fcedf7b92d3f7f1ab179e8ab55b85ee10fc1
Author: Jason A. Donenfeld <Jason@zx2c4.com>
Date:   Thu Dec 8 17:55:04 2022 +0100


    These characteristics mean that we can introduce VM_DROPPABLE, which
    has the following semantics:

    a) It never is written out to swap.
    b) Under memory pressure, mm can just drop the pages (so that they're
       zero when read back again).  <<<<<<<<<<<<<<<<<
    c) It is inherited by fork.
    d) It doesn't count against the mlock budget, since nothing is locked.
    e) If there's not enough memory to service a page fault, it's not fatal,
       and no signal is sent.

2) When kernel read back the dropped anonymous memory will trigger do_page_fault(), 
finally it will reach do_anonymous_page() and hit following line:
https://elixir.bootlin.com/linux/v4.6/source/mm/memory.c#L2750
So normally you will read zeroed memory.

Call stack happen when read back:

do_page_fault()
↓
handle_mm_fault()
↓
__handle_mm_fault()
↓
handle_pte_fault()
↓
do_pte_missing()
↓
do_anonymous_page()

> 
> -- 
> Regards,
> Li Wang

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