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b=iTiHgMO/PDzPA6rhsUoTDEqFeIfn3dQm5gBIi6RqgEFXsc2RLYEoRDgO3Gf74rhhw RijszniT+15GwHGFuCKgN2a2MVmAT1jHW8jrUAjIbDCujRScKGx3vGedA6MZyqO/Z1 4sPATzQ5P71ligD6mbDLbrTl1oo5iGmyXoHuwpDjWjRJqwZkQh9Uhrbp+lPLbKdPxf HJLMsmciaTvQnCmqQMpIw0df61P8kT/zQrci584Mfvrw0w3Yx1V+5uRRERXgdryI9S EHbGB5/zlNc0SUF2wlfEgIvT9S0Cu7qEs4qc9DeKUkKTlHfOaSkTfCv/l+8vNZhyCm aXFhNAP+Qoguw== Date: Mon, 9 Jun 2025 22:36:48 +0300 From: Mike Rapoport To: Pratyush Yadav Cc: Alexander Graf , Changyuan Lyu , Pasha Tatashin , Andrew Morton , Baoquan He , kexec@lists.infradead.org, linux-kernel@vger.kernel.org, linux-mm@kvack.org Subject: Re: [PATCH] kho: initialize tail pages for higher order folios properly Message-ID: References: <20250605171143.76963-1-pratyush@kernel.org> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: X-Rspamd-Server: rspam12 X-Rspamd-Queue-Id: 5A16540005 X-Stat-Signature: d9iqrskbr9eqxbcug5rku16h8dumwtzy X-Rspam-User: X-HE-Tag: 1749497816-144294 X-HE-Meta: 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 cqbvH3zs aSCCBmpfP5QL+80+TcAxPsHey6lbSnNhkEzqwiBfRBkKcv0E4D5BSFzvHu46WBkfD4mfffx4n/1pITnmX0TuoGcs/3/y3/OOFE987aNmVQeWloe83yAJtfXtCfK+5hU+fn0nT1z3v2ca1P86jgNdZ5x1Flckdllksz+FT46P37SHOhblxyBNoqSDAFmJQC6Nws2OzU9d4c9UCyaHKiW93YJLctK7RmSPckhQMZ53RizlRzh96c8YTGCUbGocUjJz/qGYO X-Bogosity: Ham, tests=bogofilter, spamicity=0.000000, version=1.2.4 Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: List-Subscribe: List-Unsubscribe: Hi Pratyush, On Fri, Jun 06, 2025 at 06:23:06PM +0200, Pratyush Yadav wrote: > Hi Mike, > > On Fri, Jun 06 2025, Mike Rapoport wrote: > > > On Thu, Jun 05, 2025 at 07:11:41PM +0200, Pratyush Yadav wrote: > >> From: Pratyush Yadav > >> > >> --- a/kernel/kexec_handover.c > >> +++ b/kernel/kexec_handover.c > >> @@ -157,11 +157,21 @@ static int __kho_preserve_order(struct kho_mem_track *track, unsigned long pfn, > >> } > >> > >> /* almost as free_reserved_page(), just don't free the page */ > >> -static void kho_restore_page(struct page *page) > >> +static void kho_restore_page(struct page *page, unsigned int order) > >> { > >> - ClearPageReserved(page); > > > > So now we don't clear PG_Reserved even on order-0 pages? ;-) > > We don't need to. As I mentioned in the commit message as well, > PG_Reserved is never set for KHO pages since they are reserved with > MEMBLOCK_RSRV_NOINIT, so memmap_init_reserved_pages() skips over them. You are right, I missed it. > That said, while reading through some of the code, I noticed another > bug: because KHO reserves the preserved pages as NOINIT, with > CONFIG_DEFERRED_STRUCT_PAGE_INIT == n, all the pages get initialized > when memmap_init_range() is called from setup_arch (paging_init() on > x86). This happens before kho_memory_init(), so the KHO-preserved pages > are not marked as reserved to memblock yet. > > With deferred page init, some pages might not get initialized early, and > get initialized after kho_memory_init(), by which time the KHO-preserved > pages are marked as reserved. So, deferred_init_maxorder() will skip > over those pages and leave them uninitialized. > > So we need to either also call init_deferred_page(), or remove the > memblock_reserved_mark_noinit() call in deserialize_bitmap(). And TBH, I > am not sure why KHO pages even need to be marked noinit in the first > place. Probably the only benefit would be if a large chunk of memory is > KHO-preserved, the pages can be initialized later on-demand, reducing > bootup time a bit. One benefit is performance indeed, because in not deferred case the initialization of reserved pages in memmap_init_reserved_pages() is really excessive. But more importantly, if we remove memblock_reserved_mark_noinit(), with CONFIG_DEFERRED_STRUCT_PAGE_INIT we'd loose page->private because the struct page will be cleared after kho_mem_deserialize(). > What do you think? Should we drop noinit or call init_deferred_page()? > FWIW, my preference is to drop noinit, since init_deferred_page() is > __meminit and we would have to make sure it doesn't go away after boot. We can't drop noinit and calling init_deferred_page() after boot just won't work because it uses memblock to find the page's node and memblock is gone after init. The simplest short-term solution is to disable KHO when CONFIG_DEFERRED_STRUCT_PAGE_INIT is set and then find an efficient way to make it all work together. -- Sincerely yours, Mike.