From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from mail-wm1-f73.google.com (mail-wm1-f73.google.com [209.85.128.73]) (using TLSv1.2 with cipher ECDHE-RSA-AES128-GCM-SHA256 (128/128 bits)) (No client certificate requested) by smtp.subspace.kernel.org (Postfix) with ESMTPS id 72E1A3E92AA for ; Fri, 15 May 2026 13:36:50 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.client-ip=209.85.128.73 ARC-Seal:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1778852215; cv=none; b=Oo/t66sgSuHPuAK66h24YIJMid920J2DYPVfrbEu4GxvGCRCKpbQhZrba5UdEF7bM8WwwCocdw5myQA8UYVGHX+8SgdbKDJg/YPe1ynbQbeop4PUxjHghqJbU4LN5rqe9ju2pCJYaxYlbL/cxODuY2srANCWEJdSsYy+FbjPAxQ= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1778852215; c=relaxed/simple; bh=FoNcytiKLumHYr//cNYZZmt1QoPGQxhb46UXUUVH8ZA=; h=Date:In-Reply-To:Mime-Version:References:Message-ID:Subject:From: To:Cc:Content-Type; b=MCAOZ2r1vRATvmoCyy4PXnQw/JoasURtwhSPVpA53wNqGfixa3p3AxrgyvdHAv+p7WOSYv7rdcNkYRg35PJPdy9tPNRplsZiOZZfAJQxlLrt6rMXlMGFm6YHEnkLupq4OfMeEvjQZpG9FVjOjmlOnNi0BwkXojH7iHChqIHGI2U= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dmarc=pass (p=reject dis=none) header.from=google.com; spf=pass smtp.mailfrom=flex--jackmanb.bounces.google.com; dkim=pass (2048-bit key) header.d=google.com header.i=@google.com header.b=Ej0hQptF; arc=none smtp.client-ip=209.85.128.73 Authentication-Results: smtp.subspace.kernel.org; dmarc=pass (p=reject dis=none) header.from=google.com Authentication-Results: smtp.subspace.kernel.org; spf=pass smtp.mailfrom=flex--jackmanb.bounces.google.com Authentication-Results: smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=google.com header.i=@google.com header.b="Ej0hQptF" Received: by mail-wm1-f73.google.com with SMTP id 5b1f17b1804b1-48fe3e73da6so7659405e9.2 for ; Fri, 15 May 2026 06:36:50 -0700 (PDT) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=google.com; s=20251104; t=1778852207; x=1779457007; darn=vger.kernel.org; h=cc:to:from:subject:message-id:references:mime-version:in-reply-to :date:from:to:cc:subject:date:message-id:reply-to; bh=w0CIEuptY6BMu7SFz0yWG+fGqpoNXud+HkJ9T3GbtD4=; b=Ej0hQptFgFfmiXh/68ysMxGP75BSHQ972peTNX5l5JwI+ZL6Y8v9cM8PXKVSrQtHPW JfzphLLcDIk+Ei2KkGGrO6tCP26J1ygRpsuiTvNCFX1cM+0U2E0m/Do3k2PcEYSilicy kkuseNb+zCQ+9aNjsEF/rdNqb8QuX2JRxXKKmQbmK971aHE7uRfd74Zslnc9T9E/dUda pUCBLwbAqZxsye4//NIrZ3NIT8Fu8ezenT8MgZhB93OemS6j/JOuc4ZbwAhCT1Md/nP3 KcbbJU7J9vcUdhcBhNIzPe7DReJ52bUukw0Bwb7vFcaAhAKEWziUiRt7jlhTs8Ml1tp1 DqbA== X-Google-DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=1e100.net; s=20251104; t=1778852207; x=1779457007; h=cc:to:from:subject:message-id:references:mime-version:in-reply-to :date:x-gm-message-state:from:to:cc:subject:date:message-id:reply-to; bh=w0CIEuptY6BMu7SFz0yWG+fGqpoNXud+HkJ9T3GbtD4=; b=mY1YUsK49/CbTffgtRTvxhTdsZrbKIlqCYkylCmL3OMoBiyu6cOSKTCJfhAOkhrBH9 20IlJhhcYy570xjPe7q3SxKg6gN/gASTQDmelIlbVjIoJ3VQEPwkJ9XNAZH6KPmxXNHJ AcHaBKZaa2veB8tsyZw5MoHw+bU7X17lYUfV5/eZHAr66XL18HriibsSJYxwGNcCTdVS YDftCJ/Fl85hdf1mai1aqooCH2lHBzXil3dAGRTiNae9xFC0W2LZfcDsrePuose5fZfJ jTemHuXDXcXbv38EW0atS2pl++vbKcjtWrKuKYKbP5ULK/gvud96RzIOwwJ19epKALxZ cSQg== X-Forwarded-Encrypted: i=1; AFNElJ/vmvKT641j8Ii7RBqd+WVq5hFS8PyP7dI+kjX0t704cavpTYoYW1xabjTHgDKE3RolJ8dnd3xxPwrtMqY=@vger.kernel.org X-Gm-Message-State: AOJu0YziIX2NHFeFkCIaaMex7bzxRnWCdB2b776cC9dMkwpYr1cExUKu opzAEkXW/KatYH+83yBbmCWyzhMM/Facf67tV5fQgtL4OguaNn9xoi7x11yD5+aE2xEAtM/zBlb wXSrQp+LyUXZB7g== X-Received: from wmrc26.prod.google.com ([2002:a05:600c:ada:b0:48e:6f63:7624]) (user=jackmanb job=prod-delivery.src-stubby-dispatcher) by 2002:a05:600c:a405:b0:489:1f04:96c3 with SMTP id 5b1f17b1804b1-48fe5fcded3mr53489255e9.2.1778852206796; Fri, 15 May 2026 06:36:46 -0700 (PDT) Date: Fri, 15 May 2026 13:36:43 +0000 In-Reply-To: Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: Mime-Version: 1.0 References: <20260320-page_alloc-unmapped-v2-0-28bf1bd54f41@google.com> <20260320-page_alloc-unmapped-v2-18-28bf1bd54f41@google.com> X-Mailer: aerc 0.21.0 Message-ID: Subject: Re: [PATCH v2 18/22] mm/page_alloc: introduce ALLOC_NOBLOCK From: Brendan Jackman To: "Vlastimil Babka (SUSE)" , Brendan Jackman , Borislav Petkov , Dave Hansen , Peter Zijlstra , Andrew Morton , David Hildenbrand , Wei Xu , Johannes Weiner , Zi Yan , Lorenzo Stoakes Cc: , , , , Sumit Garg , , , Will Deacon , , "Kalyazin, Nikita" , , "Itazuri, Takahiro" , Andy Lutomirski , David Kaplan , Thomas Gleixner , Yosry Ahmed Content-Type: text/plain; charset="UTF-8" On Wed May 13, 2026 at 9:43 AM UTC, Vlastimil Babka (SUSE) wrote: > On 3/20/26 19:23, Brendan Jackman wrote: >> This flag is set unless we can be sure the caller isn't in an atomic >> context. >> >> The allocator will soon start needing to call set_direct_map_* APIs >> which cannot be called with IRQs off. It will need to do this even >> before direct reclaim is possible. >> >> Despite the fact that, in principle, ALLOC_NOBLOCK is distinct from >> __GFP_DIRECT_RECLAIM, in order to avoid introducing a GFP flag, just >> infer the former based on whether the caller set the latter. This means >> that, in practice, ALLOC_NOBLOCK is just !__GFP_DIRECT_RECLAIM, except >> that it is not influenced by gfp_allowed_mask. This could change later, >> though. > > I don't think it should change later? We wouldn't want false positives > during boot, or what do you have in mind? I don't think I had anything specific in mind or any reason to _want_ to change it. But I think (??) there are reasons to clear __GFP_DIRECT_RECLAIM even if you are not atomic? Like some sort of generalisation of __GFP_NOIO/NOFS. So all I'm getting at here is: I'm using __GFP_DIRECT_RECLAIM to set ALLOC_NOBLOCK, but I think of that as a total implementation detail and these two flags should conceptually be decoupled. > I wonder if the implementation of the "not influenced" is correct though... This has been broken in several local iterations of this patchset so I would not be surprised... >> Call it ALLOC_NOBLOCK in order to try and mitigate confusion vs the >> recently-removed ALLOC_NON_BLOCK, which meant something different. >> >> Signed-off-by: Brendan Jackman >> --- >> mm/internal.h | 1 + >> mm/page_alloc.c | 29 ++++++++++++++++++++++------- >> 2 files changed, 23 insertions(+), 7 deletions(-) >> >> diff --git a/mm/internal.h b/mm/internal.h >> index cc19a90a7933f..865991aca06ea 100644 >> --- a/mm/internal.h >> +++ b/mm/internal.h >> @@ -1431,6 +1431,7 @@ unsigned int reclaim_clean_pages_from_list(struct zone *zone, >> #define ALLOC_HIGHATOMIC 0x200 /* Allows access to MIGRATE_HIGHATOMIC */ >> #define ALLOC_TRYLOCK 0x400 /* Only use spin_trylock in allocation path */ >> #define ALLOC_KSWAPD 0x800 /* allow waking of kswapd, __GFP_KSWAPD_RECLAIM set */ >> +#define ALLOC_NOBLOCK 0x1000 /* Caller may be atomic */ >> >> /* Flags that allow allocations below the min watermark. */ >> #define ALLOC_RESERVES (ALLOC_HARDER|ALLOC_MIN_RESERVE|ALLOC_HIGHATOMIC|ALLOC_OOM) >> diff --git a/mm/page_alloc.c b/mm/page_alloc.c >> index 9a07c552a1f8a..83d06a6db6433 100644 >> --- a/mm/page_alloc.c >> +++ b/mm/page_alloc.c >> @@ -4608,6 +4608,8 @@ gfp_to_alloc_flags(gfp_t gfp_mask, unsigned int order) >> (gfp_mask & (__GFP_HIGH | __GFP_KSWAPD_RECLAIM)); >> >> if (!(gfp_mask & __GFP_DIRECT_RECLAIM)) { >> + alloc_flags |= ALLOC_NOBLOCK; > > When this is called from __alloc_pages_slowpath(), gfp_allowed_mask is > already applied, so it will be influenced. ... yep. I have tried to generally refactor the flag setup in here to make these kinda mistakes harder but I didn't have any good ideas (this was when I spotted [0]). Maybe I was being too timid, I will try again. [0] https://lore.kernel.org/linux-mm/20260331-b4-prepare_alloc_pages-flags-v1-1-ea2416def698@google.com/ >> + >> /* >> * Not worth trying to allocate harder for __GFP_NOMEMALLOC even >> * if it can't schedule. >> @@ -4801,14 +4803,13 @@ check_retry_cpuset(int cpuset_mems_cookie, struct alloc_context *ac) >> >> static inline struct page * >> __alloc_pages_slowpath(gfp_t gfp_mask, unsigned int order, >> - struct alloc_context *ac) >> + struct alloc_context *ac, unsigned int alloc_flags) >> { >> bool can_direct_reclaim = gfp_mask & __GFP_DIRECT_RECLAIM; >> bool can_compact = can_direct_reclaim && gfp_compaction_allowed(gfp_mask); >> bool nofail = gfp_mask & __GFP_NOFAIL; >> const bool costly_order = order > PAGE_ALLOC_COSTLY_ORDER; >> struct page *page = NULL; >> - unsigned int alloc_flags; >> unsigned long did_some_progress; >> enum compact_priority compact_priority; >> enum compact_result compact_result; >> @@ -4860,7 +4861,7 @@ __alloc_pages_slowpath(gfp_t gfp_mask, unsigned int order, >> * kswapd needs to be woken up, and to avoid the cost of setting up >> * alloc_flags precisely. So we do that now. >> */ >> - alloc_flags = gfp_to_alloc_flags(gfp_mask, order); >> + alloc_flags |= gfp_to_alloc_flags(gfp_mask, order); > > Is it safe to just combine them? You come with ALLOC_WMARK_LOW and combine > with ALLOC_WMARK_MIN from gfp_to_alloc_flags() but these are not bit flags, > I think you end up with ALLOC_WMARK_LOW effectively. Ah, thanks, I do remember thinking about this and deciding that it was safe but I probably just misunderstood the watermark code. This makes me a bit more attracted to the idea of a struct like Gregory suggested in [1]. Then this could be captured in the type system. > Probably you need to pass the old alloc_flags to gfp_to_alloc_flags, mask > only ALLOC_NOBLOCK from it and combine with newly calculated alloc_flags. By > not recomputing ALLOC_NOBLOCK you also avoid the problem pointed out above? Nice, thanks for the pointer. > (or we decide to not use gfp flag but a new function and then it's more like > what alloc_frozen_pages_nolock_noprof() does).