On Wed, Nov 25, 2020 at 3:43 PM Minchan Kim [off-list ref] wrote:

On Wed, Nov 25, 2020 at 03:23:40PM -0800, Suren Baghdasaryan wrote:

quoted

On Wed, Nov 25, 2020 at 3:13 PM Minchan Kim [off-list ref] wrote:

quoted

On Mon, Nov 23, 2020 at 09:39:42PM -0800, Suren Baghdasaryan wrote:

quoted

process_madvise requires a vector of address ranges to be provided for
its operations. When an advice should be applied to the entire process,
the caller process has to obtain the list of VMAs of the target process
by reading the /proc/pid/maps or some other way. The cost of this
operation grows linearly with increasing number of VMAs in the target
process. Even constructing the input vector can be non-trivial when
target process has several thousands of VMAs and the syscall is being
issued during high memory pressure period when new allocations for such
a vector would only worsen the situation.
In the case when advice is being applied to the entire memory space of
the target process, this creates an extra overhead.
Add PMADV_FLAG_RANGE flag for process_madvise enabling the caller to
advise a memory range of the target process. For now, to keep it simple,
only the entire process memory range is supported, vec and vlen inputs
in this mode are ignored and can be NULL and 0.
Instead of returning the number of bytes that advice was successfully
applied to, the syscall in this mode returns 0 on success. This is due
to the fact that the number of bytes would not be useful for the caller
that does not know the amount of memory the call is supposed to affect.
Besides, the ssize_t return type can be too small to hold the number of
bytes affected when the operation is applied to a large memory range.

Can we just use one element in iovec to indicate entire address rather
than using up the reserved flags?

        struct iovec {
                .iov_base = NULL,
                .iov_len = (~(size_t)0),
        };

Furthermore, it would be applied for other syscalls where have support
iovec if we agree on it.

The flag also changes the return value semantics. If we follow your
suggestion we should also agree that in this mode the return value
will be 0 on success and negative otherwise instead of the number of
bytes madvise was applied to.

Well, return value will depends on the each API. If the operation is
desruptive, it should return the right size affected by the API but
would be okay with 0 or error, otherwise.

I'm fine with dropping the flag, I just thought with the flag it would
be more explicit that this is a special mode operating on ranges. This
way the patch also becomes simpler.
Andrew, Michal, Christian, what do you think about such API? Should I
change the API this way / keep the flag / change it in some other way?