On 21 Sep 2021, at 17:30, Peter Xu [off-list ref] wrote:

On Tue, Sep 21, 2021 at 04:08:29PM +0000, Tiberiu Georgescu wrote:

quoted

Hmmm, so if we put emphasis on the accuracy of swap info, or accuracy in
general, we need to use frontswap. Otherwise, mincore() could suffer from
race conditions, and mark pages in the swap cache as being present.

IMHO it's not a race condition, but by design.

Quotting again from the mincore() man page:

      The vec argument must point to an array containing at least
      (length+PAGE_SIZE-1) / PAGE_SIZE bytes.  On return, the least
      significant bit of each byte will be set if the corresponding page is
      currently resident in memory, and be clear otherwise.

I think "within swap cache" does mean that it still resides in memory, so it's
not violating what it's designed to me, at least from the manpage.

I agree mincore() is doing what it is designed to do. I did not express my
thoughts correctly.

quoted

Do you reckon this info (frontswap for mincore) should be present in
the pagemap docs? I wouldn't want to bloat the section either.

I don't think the type of swap matters in this document, but imho mentioning
mincore() as the alternative to fetch swap is still meaningful as that's what's
missing for pagemap right now on shmem typed memories.

Even if it cannot identify some cases between "page presents", "page stays in
page cache", or "page stays in swap cache", it'll still be good enough to me.

Yeah, it should be good enough mostly. But I feel that providing an exact
algorithm on how to separate the cases is not appropriate anymore,
as there are still exceptions, and the "perfect" algorithm needs frontswap
as a backend, which can be a fairly prohibitive precondition.

Maybe I should just note all these tools down instead and provide a brief
explanation of their usage and their downsides in the context of pagemap
information.

Thanks a lot for all suggestions! I'll cook up a v3 shortly.

Kind regards,
Tibi