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U2FsdGVkX1/fsbHZMzPEJunCzfUzysPUyKPWDYjLFL6owSiEtuvPTMH08HWO0rTKL4GNTyxNu7Xl2ZqL8ONRkUoeEVsyQ9m1JgHT0OetWd7N9TaI3jEZXt8IFXPt3dfTq2/boBiDjzMkFuV9+J+CYbpugHr6b7ZB1nLMCUYgV1I7nTDy2rOxQJ3EEJO3scy5mfD3S7nZ39fC9l7w4CFf1gIo79myXBPXqIRc31+33SX2Hg1ZGNqDnv9UweCR+54I82GKV8+Ypftb9aru7EPqOPTU/iiOR5RDOvbRJ78ffECtkXn7FHB2QWmgYFTh9ghERwFUbWts338lls4pl5kneeOh91ucS1yMUmYR8jj5eHVDRuiaSLIbCSxCBTYeuavOZEas7eb2KVK7/lWpSaDGuyvcVYoUbfGwZbPH1mdXCj8PodhqYu/4BgkML21KFVWpVqqAvNYyy64Um730NnwYrGxiEwTlMEcJcWT1SfYlpq/VmerF8Cq9fN1FvAi37KhLixqZ+Gdd7gpjh5Mg/tuBHVAEwHqJ1ow1BYeNHH9iKVGD5lXTpWWXC8k4VhYQ4aZsLHySIhIExBe7RtmyIF8K2KVUMGHu7+4eEST2EfsqJ8G79xT/BFW5b0yUe3S7R9ZqQDpQQIG/tppSMtk17/gSIRl8H2m5M57u0PehnmPGsf/6RyO5FvQ1zrDg61o9XM9JcY9L23vSEH0sNfCt4Flp4qG9RON9Od/mT2dRhepwo7+Jn8FCueVIJwsUhlbO0tBZY+wLuPJZLaG+OprFXq/UsMFbleYphx+X15x2fMy+GjiABWlqocTDHwig1cnt6hID8pAzDpl4LrUiOYXtA2hw2ZFFtI+h/DJHzRKFjuyzcylkmUUKhbZonPAZv4IPM9eIpkNqIuKsvavKuUscSDJ1sSrxhs0hgTlSMuA3HMdE54bhajHjEt4Lnpqxa8EwW7OYqQOuAPYFYiIIB1fAu2P Y63Qle3d 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Or someone could use page->private on tail pages and free non- = zero ->private that way. >>>> >>>> [...] >>>> >>>> >>>> Thanks. >>>> >>>> >>>> Right. >>>> >>>> >>>> Right. And whether it is okay to have any tail->private be non-zero. >>>> >>>> >>>> Ideally, I guess, we would minimize the clearing of the ->private fiel= ds. >>>> >>>> If we could guarantee that *any* pages in the buddy have ->private cle= ar, maybe >>>> prep_compound_tail() could stop clearing it (and check instead). >>>> >>>> So similar to what Vlasta said, maybe we want to (not check but actual= ly clear): >>>> >>>> >>>> diff --git a/mm/page_alloc.c b/mm/page_alloc.c >>>> index e4104973e22f..4960a36145fe 100644 >>>> --- a/mm/page_alloc.c >>>> +++ b/mm/page_alloc.c >>>> @@ -1410,6 +1410,7 @@ __always_inline bool free_pages_prepare(struct p= age *page, >>>> =C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0= =C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2= =A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0 } >>>> =C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0= =C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0 } >>>> =C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0= =C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0 (p= age + i)->flags.f &=3D ~PAGE_FLAGS_CHECK_AT_PREP; >>>> +=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2= =A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0 set_page_pr= ivate(page + i, 0); >>>> =C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0= =C2=A0=C2=A0=C2=A0=C2=A0 } >>>> =C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0 } >>> >>> Thinking again, maybe it is indeed better to rework the code to not all= ow freeing pages with ->private on any page. Then, we only have to zero it = out where we actually used it and could check here that all >>> ->private is 0. >>> >>> I guess that's a bit more work, and any temporary fix would likely just= do. >> >> I agree. Silently fixing non zero ->private just moves the work/responsi= bility >> from users to core mm. They could do better. :) >> >> We can have a patch or multiple patches to fix users do not zero ->priva= te >> when freeing a page and add the patch below. > > Do we know roughly which ones don't zero it out? So far based on [1], I found: 1. shmem_swapin_folio() in mm/shmem.c does not zero ->swap.val (overlapping with private); 2. __free_slab() in mm/slub.c does not zero ->inuse, ->objects, ->frozen (overlapping with private). Mikhail found ttm_pool_unmap_and_free() in drivers/gpu/drm/ttm/ttm_pool.c does not zero ->private, which stores page order. [1] https://lore.kernel.org/all/CABXGCsNyt6DB=3DSX9JWD=3D-WK_BiHhbXaCPNV-GO= M8GskKJVAn_A@mail.gmail.com/ > >> The hassle would be that >> catching all, especially non mm users might not be easy, but we could me= rge >> the patch below (and obviously fixes) after next merge window is closed = and >> let rc tests tell us the remaining one. WDYT? > > LGTM, then we can look into stopping to zero for compound pages. > >> >> >> diff --git a/mm/page_alloc.c b/mm/page_alloc.c >> index 24ac34199f95..0c5d117a251e 100644 >> --- a/mm/page_alloc.c >> +++ b/mm/page_alloc.c >> @@ -1411,6 +1411,7 @@ __always_inline bool free_pages_prepare(struct pag= e *page, >> } >> } >> (page + i)->flags.f &=3D ~PAGE_FLAGS_CHECK_AT_PREP; >> + VM_WARN_ON_ONCE((page + i)->private); >> } >> } >> if (folio_test_anon(folio)) { >> @@ -1430,6 +1431,7 @@ __always_inline bool free_pages_prepare(struct pag= e *page, >> >> page_cpupid_reset_last(page); >> page->flags.f &=3D ~PAGE_FLAGS_CHECK_AT_PREP; >> + VM_WARN_ON_ONCE(page->private); >> page->private =3D 0; >> reset_page_owner(page, order); >> page_table_check_free(page, order); >> >> >> Best Regards, >> Yan, Zi > > > --=20 > Cheers, > > David Best Regards, Yan, Zi