From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from out-188.mta0.migadu.com (out-188.mta0.migadu.com [91.218.175.188]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by smtp.subspace.kernel.org (Postfix) with ESMTPS id A82FC22B8B6 for ; Fri, 27 Mar 2026 16:53:46 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.client-ip=91.218.175.188 ARC-Seal:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1774630428; cv=none; b=kES58EolnqvL0HI6x41LzF0yZx1ndLBpwUIfPBDTtGnv0kO+1h/HlCtKszVW6ew55CgglHKo3F3+XOKHTHtx2m9KEVdKThrNk5SlLG8HcrPmQsDqxMRi0rO4QpDe+wYpil1rxkDaQ8IEAd343b8LVeP2Bj1248Sjbn6TSTuWBEY= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1774630428; c=relaxed/simple; bh=Gl6KmMcq+tjSc4jpZX6oUnfWp7Ff3JAsTfx9UiwXWgY=; h=Message-ID:Date:MIME-Version:Subject:To:Cc:References:From: In-Reply-To:Content-Type; b=k2yhQaDW7cXa84n3FgrP1G7trhtWFeRM8RFqAeep0roRMCWDFbVH9uilPvbzpwIXJxFeLQbMbDQtfKrTGVL0tGH36SpIi5lMMt07Pz60UsbOrY96bEJTqbAgVGC7VBTfnq4n0iNm8qIYakxBjdFzFhbFRK8vtzJQX+YSCrnlFZ8= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dmarc=pass (p=none dis=none) header.from=linux.dev; spf=pass smtp.mailfrom=linux.dev; dkim=pass (1024-bit key) header.d=linux.dev header.i=@linux.dev header.b=f75mPENZ; arc=none smtp.client-ip=91.218.175.188 Authentication-Results: smtp.subspace.kernel.org; dmarc=pass (p=none dis=none) header.from=linux.dev Authentication-Results: smtp.subspace.kernel.org; spf=pass smtp.mailfrom=linux.dev Authentication-Results: smtp.subspace.kernel.org; dkim=pass (1024-bit key) header.d=linux.dev header.i=@linux.dev header.b="f75mPENZ" Message-ID: <0725ce97-b8a3-47c9-952f-7b512873cc35@linux.dev> DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=linux.dev; s=key1; t=1774630424; h=from:from:reply-to:subject:subject:date:date:message-id:message-id: to:to:cc:cc:mime-version:mime-version:content-type:content-type: content-transfer-encoding:content-transfer-encoding: in-reply-to:in-reply-to:references:references; bh=I9csAY2Lvawgp7Rsam87YjKnm7rldK4hTm6EIfaKTPo=; b=f75mPENZ/0pogd7iZftgXzxU1U3uP/uDnUH51xNQ/Zl5ECBs158ZiTywN4eeQRE3ZbgIbD SOe/t2tapJrZ/LL8Ctf7vJeDgJkGGx5TgRBO6QFlILAZ2e8der8tSRw0PhdSMLZ2+GanQJ v6cbU9Vsavrir2wRkkI4EJWCx3V2d4c= Date: Fri, 27 Mar 2026 12:53:34 -0400 Precedence: bulk X-Mailing-List: linux-fsdevel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Subject: Re: [PATCH v2 3/4] elf: align ET_DYN base to max folio size for PTE coalescing Content-Language: en-GB To: WANG Rui Cc: Liam.Howlett@oracle.com, ajd@linux.ibm.com, akpm@linux-foundation.org, apopple@nvidia.com, baohua@kernel.org, baolin.wang@linux.alibaba.com, brauner@kernel.org, catalin.marinas@arm.com, david@kernel.org, dev.jain@arm.com, jack@suse.cz, kees@kernel.org, kevin.brodsky@arm.com, lance.yang@linux.dev, linux-arm-kernel@lists.infradead.org, linux-fsdevel@vger.kernel.l, linux-fsdevel@vger.kernel.org, linux-kernel@vger.kernel.org, linux-mm@kvack.org, lorenzo.stoakes@oracle.com, mhocko@suse.com, npache@redhat.com, pasha.tatashin@soleen.com, rmclure@linux.ibm.com, rppt@kernel.org, ryan.roberts@arm.com, surenb@google.com, vbabka@kernel.org, viro@zeniv.linux.org.uk, willy@infradead.org References: <20260320140315.979307-4-usama.arif@linux.dev> <20260320160519.80962-1-r@hev.cc> X-Report-Abuse: Please report any abuse attempt to abuse@migadu.com and include these headers. From: Usama Arif In-Reply-To: <20260320160519.80962-1-r@hev.cc> Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 8bit X-Migadu-Flow: FLOW_OUT On 20/03/2026 19:05, WANG Rui wrote: > Hi Usama, > > On Fri, Mar 20, 2026 at 10:04 PM Usama Arif wrote: >> diff --git a/fs/binfmt_elf.c b/fs/binfmt_elf.c >> index 8e89cc5b28200..042af81766fcd 100644 >> --- a/fs/binfmt_elf.c >> +++ b/fs/binfmt_elf.c >> @@ -49,6 +49,7 @@ >> #include >> #include >> #include >> +#include >> >> #ifndef ELF_COMPAT >> #define ELF_COMPAT 0 >> @@ -488,19 +489,51 @@ static int elf_read(struct file *file, void *buf, size_t len, loff_t pos) >> return 0; >> } >> >> -static unsigned long maximum_alignment(struct elf_phdr *cmds, int nr) >> +static unsigned long maximum_alignment(struct elf_phdr *cmds, int nr, >> + struct file *filp) >> { >> unsigned long alignment = 0; >> + unsigned long max_folio_size = PAGE_SIZE; >> int i; >> >> + if (filp && filp->f_mapping) >> + max_folio_size = mapping_max_folio_size(filp->f_mapping); > > From experiments (with 16K base pages), mapping_max_folio_size() appears to > depend on the filesystem. It returns 8M on ext4, while on btrfs it always > falls back to PAGE_SIZE (it seems CONFIG_BTRFS_EXPERIMENTAL=y may change this). > This looks overly conservative and ends up missing practical optimization > opportunities. mapping_max_folio_size() reflects what the page cache will actually allocate for a given filesystem, since readahead caps folio allocation at mapping_max_folio_order() (in page_cache_ra_order()). If btrfs reports PAGE_SIZE, readahead won't allocate large folios for it, so there are no large folios to coalesce PTEs for, aligning the binary beyond that would only reduce ASLR entropy for no benefit. I don't think we should over-align binaries on filesystems that can't take advantage of it. > >> + >> for (i = 0; i < nr; i++) { >> if (cmds[i].p_type == PT_LOAD) { >> unsigned long p_align = cmds[i].p_align; >> + unsigned long size; >> >> /* skip non-power of two alignments as invalid */ >> if (!is_power_of_2(p_align)) >> continue; >> alignment = max(alignment, p_align); >> + >> + /* >> + * Try to align the binary to the largest folio >> + * size that the page cache supports, so the >> + * hardware can coalesce PTEs (e.g. arm64 >> + * contpte) or use PMD mappings for large folios. >> + * >> + * Use the largest power-of-2 that fits within >> + * the segment size, capped by what the page >> + * cache will allocate. Only align when the >> + * segment's virtual address and file offset are >> + * already aligned to the folio size, as >> + * misalignment would prevent coalescing anyway. >> + * >> + * The segment size check avoids reducing ASLR >> + * entropy for small binaries that cannot >> + * benefit. >> + */ >> + if (!cmds[i].p_filesz) >> + continue; >> + size = rounddown_pow_of_two(cmds[i].p_filesz); >> + size = min(size, max_folio_size); >> + if (size > PAGE_SIZE && >> + IS_ALIGNED(cmds[i].p_vaddr, size) && >> + IS_ALIGNED(cmds[i].p_offset, size)) >> + alignment = max(alignment, size); > > In my patch [1], by aligning eligible segments to PMD_SIZE, THP can quickly > collapse them into large mappings with minimal warmup. That doesn’t happen > with the current behavior. I think allowing a reasonably sized PMD (say <= 32M) > is worth considering. All we really need here is to ensure virtual address > alignment. The rest can be left to THP under always, which can decide whether > to collapse or not based on memory pressure and other factors. > > [1] https://lore.kernel.org/linux-fsdevel/20260313005211.882831-1-r@hev.cc > >> } >> } >> >> @@ -1104,7 +1137,8 @@ static int load_elf_binary(struct linux_binprm *bprm) >> } >> >> /* Calculate any requested alignment. */ >> - alignment = maximum_alignment(elf_phdata, elf_ex->e_phnum); >> + alignment = maximum_alignment(elf_phdata, elf_ex->e_phnum, >> + bprm->file); >> >> /** >> * DOC: PIE handling >> -- >> 2.52.0 >> > > Thanks, > Rui