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    SubjectRe: [PATCH] block/psi: make PSI annotations of submit_bio only work for file pages
    On Wed, Mar 30, 2022 at 08:54:09AM -0700, Christoph Hellwig wrote:
    > On Wed, Mar 30, 2022 at 11:45:56AM -0400, Johannes Weiner wrote:
    > > > FYI, I started redoing that version and I think with all the cleanups
    > > > to filemap.c and the readahead code this can be done fairly nicely now:
    > > >
    > > > http://git.infradead.org/users/hch/block.git/commitdiff/666abb29c6db870d3941acc5ac19e83fbc72cfd4
    > >
    > > Yes, it's definitely much nicer now with the MM instantiating the
    > > pages for ->readpage(s).
    > >
    > > But AFAICS this breaks compressed btrfs (and erofs?) because those
    > > still do additional add_to_page_cache_lru() and bio submissions.
    >
    > In btrfs, add_ra_bio_pages only passed freshly allocated pages to
    > add_to_page_cache_lru. These can't really have PageWorkingSet set,
    > can they? In erofs they can also come from a local page pool, but
    > I think otherwise the same applies.

    It's add_to_page_cache_lru() that sets the flag.

    Basically, when a PageWorkingset (hot) page gets reclaimed, the bit is
    stored in the vacated tree slot. When the entry is brought back in,
    add_to_page_cache_lru() transfers it to the newly allocated page.

    add_to_page_cache_lru()
    filemap_add_folio()
    __filemap_add_folio(mapping, folio, index, gfp, &shadow)
    *shadow = *slot
    *slot = folio
    if (shadow)
    workingset_refault(folio, shadow)
    folio_set_workingset()

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